Thank you everyone for the replies!
It is actually a Lyman, left handed, and that is the original factory plug, which from what I understand is an oddball metric size, and difficult to source.
I may just try to go up to a 3/8ths. It may be ugly, but it’s a hunting rifle. Safety and function are my primary concerns over aesthetics.
I cringe a little when thinking about trying to get that factory plug out of there. Almost think I’d rather get a Green Mtn and rebarrel the thing.
Just had to try to make it faster
Couldn’t leave well enough alone...