I’m told this is a Russian 45MM cannon, and that it has a de-milled breach ring. It is best to inspect and do some research on the details of this piece.
The 45mm gun bearing 53-K was a combination of a modified carriage of the 37mm anti-tank gun model 1931 with a 45 mm barrel. The resulting light semi-automatic anti-tank gun was adopted for Red Army service in 1937, and thus known as “45 mm anti-tank gun M1937”. These guns were used in the first stage of the German-Soviet War, but their anti-armor capabilities allowed them to fight successfully only with German light tanks and armored personnel carriers. Early models of the Panzer III and Panzer IV could also be knocked out at close range, but this put Soviet artillery men in greater danger. Due to these circumstances, model 1937 guns were replaced with the all-new design, the more powerful model 1942. The mass production of outdated model 1937 guns was stopped in 1943.