Bob,
Well you got me doing your homework again for you, since you know I'm laid up with the shoulder problems..........Taking advantage of the crippled guy.... Geeezzzee.
Your's looks like those small Repro Bomb Shell stamps.
The common one I've see have no flame licks and are often on stocks that have the boxed RIA over EB stamped stocks, So is it something RIA added? I'm not sure, because we've run across stocks that had no acceptance stamps/CC's but had the bomb by the recoil plate, Maybe replacement stocks when accepted were stamped with it?
I know Frank 'The Foot' Derrico has run across quite a few also.
http://www.milsurps.com/showthread.php?t=10143
https://www.milsurps.com/showthread.php?t=22377