Shaggy, thanks for reminding me about the Colt nomenclature series in SAR. I went back and looked them up. They show the 609 and the 610B.
My scanner is not working right now but the 609 is marked with the Prancing Horsey logo and COLT AR-15, below that PROPERTY OF U.S. GOVT., below that COMMANDO, below that CAL5.56 MM, below that SERIAL XXXXXX.
There's a short paragraph below-
'This is the original "xm177" model of the M16 submachine gun. Original 609's are marked "commando" as in the issustration, and they are also U.S. Property Marked. Colt went back to the full pistol grip on the Commando. The "E1" Army version had the forward assist, and the straight XM177 do not-this was the Air Force model.
The 610B is marked with the Prancing Horsey next to COLT AR-15, below that is an open space (where COMMANDO was marked above), below that MOD. 610B, below that cal 5.56 MM, below that SERIAL XXXXXX
Also a short paragraph below this image-
The 610B toolroom guns were four position three shot burst submachine guns. The 610 series had no forward assist, and the regular 610 did not have a burst mechanism.
Hope this helped.