[Stellan Bojerud (RIP)]

I think you are right.

Looking closer on the wheels and the outriggers I tend to think that it is a 40 mm US-made Bofors. Lend-lease I suppose. In such case made by Chrysler Coorporation. Chrysler made guns were the only type that had the welded version of the original riveted MK 1 type which if I remember right was US type M 2.

I was confused because the gun has russian sights. Could have been added later to facilitate training.