What you have there is a cartridge belt buckle that was made for wear on the various types of cartridge/ammo belts used from the 1920's through the early 1940s. I have a good size collection of Manchu buckles and have one of these along with the 1926 original that is worn on the Sam Brown belt from that time period. I have acquired a good amount of info on these buckles and posted it on my web site here Manchu Buckle - C Co 3/9 MANCHU "The Chosen Few" .
I don't see these popping up very often. They are more rare then the 1926 originals which I have seen come up for sale more often then the cartridge buckle. That is a good find on your part.