Help with Thompason Magazine

[browningautorifle]

That one's good.

not round but slightly oval, it is a repro Japanese made mag I believe

The oval hole is for the 1927 semi auto Auto Ord or Numrich guns, they fire from a closed bolt and differ completely inside from GI. They were standard mags hogged up so you had to use those, you couldn't just grab mags and take them to a gunfight and win. The drum still fit but there aren't many of those except those of us serious enough to have examples.

Due to no markings and condition what would be the best way to come up with a value?

Honestly? I'd only go about $20 USD on that one. It's a well worn mag...