I was on active duty at Camp Pendleton in 1991. Let me just say this about that
There was a DRMO (Defense Reutilization and Marketing Office) site at Pendleton. In those days DRMO acted as sort of the junkyard and/or pawn shop for military bases. Property that was worn out, broken, outdated and being dropped from the system, excess, etc. would be transferred to DRMO. Then DRMO would hold auctions open to the public, where people could bid on the property. However, the property whatever it might be, a pallet load of worn out ALICE packs, broken radios, scrap pipe, whatever, had to be on a list as approved for release to the public.
Functional weapons were NOT releasable to the public and would not have been auctioned off to the public. The USMC would have sent an old shotgun probably to one of the two USMC logistics depots at Barstow or Albany for disposition. More than likely from there they would have been put in long term storage, transferred to the Army for storage, or destroyed.
AFAIK the only mil/gov't agency with the authority to transfer a firearm direct to the public is CMP.
I'd be leery of those markings unless you can find documented proof that it's legit, which I don't think you're going to find.
Something else you might try is sending a PM to Gus Fisher, he was a USMC armorer and I believe also at Pendleton around that time so he would probably know exactly what the USMC would have done with an old shotgun being dropped from the system. HTH