While I don't claim to be an AK74 expert over the years I have picked up some working knowledge and wanted to put down a quick primer here. I have always considered myself an amateur AK owner and enthusiast, and my opinions are still written from that perspective. There are many many enthusiasts who have considerably more knowledge than I do, and this page is an acknowledgement of so many people's work.


Please be sure to check out the absolute best reference on AK rifles which covers this subject in amazing detail: Avtomats In Action AK 74 Variants

Also you can download and review the extremely detailed AK74 parts and components

Ekie, Tantal and many others have contributed to creating an incredible reference on AKs. It is a site for the true AK collector and has detail that is invaluable for clone builds, as well as show casing the incredible talent as contributors build amazing replica's. There is no better AK reference site

Differences in AK74 Patterns - Simple Overview

RPK74M

Fullsize standard AK74's are generally split into 3 major era's of production patterns

I will provide some details here as it relates to my builds but to be clear however, this is just touching on the topic and is really just a quick review of the highlights of each era.

Original: 1974 -1986 - AK74, AK-74N, AKS-74, AKS-74N - Laminate furniture
Mid: 1986 to 1990ish - AK74, AK74N, AKS-74, AKS74N - Plum furniture
Modern: 1990 and later  - AK-74M - Black polymer furniture

AK74 denotes fixed stock rifle with no optic rail

AK74N is a fixed stock with an optic rail
AKS-74 is a folding stock rifle with no optic rail

AKS-74N is a folding stock rifle with an optic rail
AK-74M is a rollup of 20 years of 74 manufacturing improvements, and includes a folding stock and optic rail standard on every rifle

Service AK74M rifles are all folding stock with pistol grip plates, 5.5mm folding hardware and have a side rail optic mount installed standard as part of the M (Modernizirovanniy) designation. Previously called AK74 (fixed stock, no rail), AK74N (fixed stock with side rail), AKS74 (folding stock, no side rail) or AKS74N (folding stock with side rail mount), the 90s era cumulative production improvements resulted in the rifle simply being called the AK74M.

There are training rifles which have the black polymer furniture and 74M features but are fixed stock, however these are not in front line service as I understand it.

There are also 4 main physical variants of the AK74 pattern rifle:

AK74 16 inch barrel full size rifle
AKS74U (AKSU) (Commonly called Krinkov in the US, but no one knows why) - 8 inch barrel, short gas system
RPK74 - 20 inch barrel, designed as a squad light machine gun
AK105 - 12 inch barrel, similar to the M4 vs the M16 rifle

After 1992 when the AK74M became the standard issue 5.45x39 rifle for the Russian army, the Russians also developed export versions of the AK74M chambered in popular calibers:

AK100 series:
AK101 = 5.56x45 16" Barrel
AK102 = 5.56x45 12"Barrel
AK103 = 7.62x39 16" Barrel
AK104 = 7.62x39 12" Barrel
AK105 = 5.45x39 12" Barrel

AK12 series:

As part of a modernization request by the Russian army in the mid to late 2000s, Izhmash developed the AK12 series of rifles which are essentially AK74M rifles with a modified gas tube, top cover and receiver, but generally keeping most of the AK74M compatibility. For example, the bolt and bolt carrier are standard AK74 parts and custom builds here in the US typically use surplus Bulgarian carriers and front trunnions due to the fact that a full AK12 parts kit is impossible to find. Without going into too much detail I will say that the AK12 is actually a pretty neat rifle and doesn't suck nearly as bad as the Internet makes it out to. More on that subject in the AK12 write up

As with the AK100 series being a derivative of the AK74M, the AK12 series comes in different calibers:

AK12 = 5.45x39
AK15 = 7.62x39
AK19 = 5.56x45

Check the Rifles section for more details

Fullsize AK74 differences:

If you are considering modifying a fixed stock SLR or SGL rifle to be period correct the simple guideline is to use Bulgarian parts for the SLR series and Russian parts for the SGL (and Vepr) series. These rifles have become very collectible and if you are taking the time and effort to modify them from the sporter configuration into the more military looking clone it is wise to spend a little more time and money to be sure the conversions or upgrades are done properly

Also of note is that the Bulgarians never modernized their original Russian supplied AK74 tooling and so the current production SLR series are actually very close to 70's and 80s production rifles.

Muzzle Device Patterns:
Top AK74M long collar (In service 1990+)
Middle: 1988 Faceted (1986-1990ish)
Bottom: Early short collar (1974-1985ish)

Both Russian and Bulgarian are 24x1.5R, vs AKM 14x1L

Also note that Yugoslavian rifles use a similar design but the threads are 26x1.5L (slightly larger, timed differently and left instead of right. They will not fit Russian and Bulgarian FSBs)

Folding Stock Trunnions:

Top: Modern 5.5mm with military hinge pin (In service 1990+)
Middle Modern 5.5mm with civilian hinge pin
Bottom: 4.5mm (Russian 1974-1990, Bulgarian 1974 to present)

The difference in hinge pins is mostly cosmetic but military pins are solid and civilian pins have a smaller middle diameter to accommodate the government mandated linkage that prevents the rifle from firing with the stock folded

***
It is not a good idea to use a 4.5mm folding hinge on a modern Saiga type rifle, so be mindful if you having fixed stock guns converted to folders. There are clear and obvious differences between the newer AK74M/Saiga type rifles vs the older Bulgarian and Russian AK74. Saiga's should use the Russian 5.5mm hardware and never the older 4.5mm hardware. There are plenty of folding stock options for both pin sizes, so you won't be limited to an incorrect pin size because you want to use a specific stock
***

The AK74M switched to a different stock angle of 4 degrees instead of the older 6 degrees. See below for ThirtyCal's photo, who is an accomplished builder, and is also featured on Avtomats in Action - AK103. His AK103 build is amazing and his attention to detail is one of the best in the hobby.
It is not easy to adapt a 4.5mm trunnion to the 5.5mm stocks and vice versa, so if you are considering a folding stock build it is best to start with the period correct hardware

AK Front Trunnion Rivet Pattern
Russian AK74M rifles switched to 3 rivets in the 90s (The large 3rd rivet is a wear rivet to assist the bolt when it locks into battery)
Russian pre 90's and all Bulgarian rifles use the older 2 trunnion rivet pattern
Top and Middle: Russian SGL31
Bottom: Bulgarian SLR105

Side rail patterns are different as well as you can see above

Furniture is also different for the 3 eras:
Russian laminate for the early pattern rifles, plum for the mid series and black polymer for the modern rifles

Top: Laminate (70s to mid 80s)
Bottom: Black Polymer (early 90s onward)

Plum (Mid 80s to late 80s): Note that Bulgaria did not actually manufacture plum furniture, so what is called Bulgarian plum is actually Soviet made. You will often see the iconic Russian silver proof marks on 'Bulgarian' furniture

K-Var US made plum furniture is also a different composition and color. Very good quality parts which are also 922r compliant, but not a direct color match.

Top: K-Var US
Middle: Russian
Bottom US Black Poly (NATO length)

There are 4 primary Russian AK74 magazine patterns:
(Photo courtesy of Avtomats In Action)

Example of True Black modern magazines (top) compared to early and late plum (bottom)

Here you can see the plum follower on the left and the true black on the right. Late plum magazines often look black depending on the lighting conditions. Always ask to see a good photo of the follower which will help determine if it's a plum mag or an actual true black

AK12 magazines are compatible with standard AK74 rifle magwell dimensions and should be backwards compatible for most properly built AK74 pattern rifles. There are a few versions on the market. The military mags will have glow in the dark followers and fully loaded pins that will pop out of the floor plate to indicate there are rounds inserted. CX mags typically accompany the CX non-firing replica rifles. I'll provide more detail in the AK12 rifle description

Here you can see a Russian Saiga on top, a Bulgarian kit built 74 and an AK12 clone all with the modern AK12 5.45 mag. I have not tested these on all AK74 variants, but they should work properly