From what it looks like, part of the modle number is missing.
Here is ArmaLites' information to help identify rifles better...
Some things here might not apply to your rifle that you are looking at, but it's the whole list.
My question is this: Is this an Armalite rifle from it's numbers/letters? It looks like it is. A good part number might look like AR10A4CB. Newre ones will look like this AR10A4CBF
If so, is it an A4 or? unsure on that, part of the modle is missing
Also if it's older were there any problems with this model ( if it's an Armalite)? Some of the older ones had larger gas port holes.
What mags can it use? Depending if any upgrades have been done to it, it will use Gen I mags. It can use the Gen II mags but will not fully operate the bolt stop (unless that has been upgraded to the Gen II mod)
10 = 7.62 X 51 / .308 (NATO chambered for all stainless steel and chrome lined barrels).
15 = 5.56 X 45 / .223 (NATO chambered for all CMV chrome lined barrels, Wylde chamber for all stainless steel barrels).
243 = .243 caliber.
260 = .260 Remington.
30 = AR-30 Rifle (.300 Winchester Magnum 30M300, .308 Winchester 30M308, and .338 Lapua 30M338).
338 = .338 Federal.
50 = AR-50 - 50BMG (Browning Machine Gun).
6.8 = 6.8 SPC II chamber.
708 = 7MM-08 caliber.
7.62X39 = 7.62X39MM caliber.
A2 = Integrated carry handle with A2 front sight.
A4 = Flat top picatinny upper receiver and gas block.
A4 and A2 (in same part number) flat top upper receiver and A2 front sight.
B = Black furniture (no "B" is standard green furniture).
C = Carbine 16" barrel.
F = Forward assist.
K = Doesn't come with detachable carry handle which was standard at one time for these particular models.
L = AR-50 Left handed model.
LEC = Law Enforcement Commercial (M203 step down barrel and 6" "shorty" gas system).
N = National Match 2 stage trigger.
NM = National Match SST barreled rifle with NM 2 stage trigger.
S = Stainless steel barrel.
SPR1CB - SPR MOD 1 - 8 inch mid-length carbine gas system
SPR1LB - SPR MOD 1 - 6 inch "shorty" carbine gas system
T = Target stainless steel barrel.
-2 = AWB (Assault Weapons Ban) legal firearm (NJ, NY, MA, CT).
1913 = ALOSTYR 2 piece non-free float handguard (black)
