You should find the larger 7/8ths head with an "circled A" on it. ("M" is correct also)
I looked through my Corvette Catalogs, and I don't see anyone making a repro version.
I also looked through my supply of drain plugs, and I did not find a correct one for you either.
Good luck, it is going to be difficult. I see them on eBay every now and then, but they get expensive.
I never really took those from the yards, I felt guilty about letting the oil run out on to the ground.
Quite a few people have asked me over the years for those.
Lucky my cars have the original ones still installed.
Long Island Corvette Supply has the right size, but no mention of head marking - $2.50 (likley meaning it is not correct)