I believe you may have an early lend lease revolver given to England in 40 or 41. Many were distributed through out the common wealth and may or may not have British proof marks. I have a lend lease Victory Model that came back from Australia without proof marks. The British requested these revolvers with 5 inch barrels, in the .38 S&W caliber with a 200 grain projectile. You can also try using a copper penny to remove some of the rust.

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Ed