Just picked up a used Colt 6940 Monolithic Rail AR. Can anyone tell me the purpose of this screw on the upper receiver? Attached File Never seen one before.
It retains the steel bearing plate for that cam pin, which sits just behind the cam pin pocket. It was to prevent wear/peening in that location in Colt’s piston uppers.
You mentioned that you had not seen the cam pin plate modification before. There are many examples out there without the modification. There were early production 6940-series firearms manufactured without the cam pin plate.
As mentioned previously, the design change was for the LE6940P but, for ease of production, was implemented across all 6940-series upper receivers.