I also have one-has belgian proof on rear face of cylinder. Hand spring is broken so cylinder won't rotate on it's own. only works in single action-will work in double action if hammer is set at half cock first.
44-40 or 44 russian?
look in the cylinder chambers. If you see a ridge or ring about 2/3s the way down (from the rear of the cylinder) it's a 44 russian cal. if the chambers are smooth it's a 44-40. most were 44 russian.
S&W 44 1880
I have a revolver with the same markings,with a 4-# SN,6" bbl. I tried a 44 mag round in the cylinder and though the round only fit in 2/3s of the way,diameterwise it was a match. Possibly 44-40?