[Trapshooter1100]

My 1979 trap t has the diamond in the cap, my 2014 sporting 12 has the "r a"

 

[skeet_man]

With all the variants of 1100 made over the years, that is definitely not a reliable way to date a gun.

They started with the gun in 1963, and were used on one form of 1100 or another at least into the 2000s (the Classic Field being the latest model that comes to mind that had that grip cap).

 

[Song Dog]

To add what "skeet man" posted Diamond grip caps production ran from 1963 through 1987 the intro of the 11=87 when the grip cap was changed.