m12 riot barrel crossed out full to cyl marking

Hey saw a m12 US marked riot 12Ga today. I know a little bit about shotguns, but not a lot. I do know the m12's were marked cyl on the barrel. But this one is marked full, then crossed out and marked cyl underneath. It almost looks factory, or very well done by maybe an arsenal?

It is a rebuild. It is parked and RIA marked on the stock, the barrel does have the winchester stamp on top with flaming bomb and all the markings on the shotgun look correct.

Serial number was like 960,xxx or something like that. And both serials matched.

Is the barrel maybe a arsenal replacement, or a commercial barrel reworked during wartime for the govt, or just a bubba'd special? Thanks guys!

Also anyway to tell if a M12 was used by the Marines? I read the M12 was used extensively by the USMC more so than the army. Just curious.

Steve

...the use of jets made the use of long bbl'd full choke shotguns as anti-aircraft training obsolete. Many of these guns were overhauled and refinished at the arsonals into shorter , more usefull riot guns. Lining out and restamping the choke markings , stamping the stock with the arsonals rebuild mark , sanding off the old stock marks , and appling a park finish are signs of this being done.
HTH , Chris