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
Fullsize standard AK74's are generally split into
3 era's of major production patterns and I will
provide some details here as it relates to my builds.
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
Mid: 1986 to 1990
Modern: 1990+
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 variants of the AK74 pattern rifle:
AK74
AKS74U (AKSU) (Commonly called
Krinkov in the US, but no one knows why)
RPK74
AK105
Check the Rifles
section for more details
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.
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 
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
Bottom: Polymer
Plum
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 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
