This is a part of the drim magazine for the MG34/42 series of German MGs, I believe. When I was collecting that caliber, I classified it as a special purpose dummy round caliber 7.9 x 57, regardless of its dimensions. My understanding is thaqt the design of some drum magazines would preclude them from feeding the last two or three cartridges. Therefore, rather than having live rounds in the magazine basically forever, the magazines were designed with the appropriate number of “dummy” rounds built into the mechanism. That way, all live rounds feed properly. I believe that in some of the magazines, all of the “dummy rounds” are not necessarily identical to each other. Unfortunately, while I had several of these taken from destroyed magazines in my collection, I have no experience with these German MGs, other than having once fired an MG34, or with their magazines, and since I could never own one being in California, I have never studied them. Therefore, consider the above answer as a gross over-simplification of the subject.
I said that they are part of the MG34/42 series of drum magazines. In truth, I don’t even know if the magazines for those two MGs are interchangeable with each other, nor do I know if
these “dummies” were used in other magazines for other German MGs.
I am sure someone far more conversant with these things than am I will chime in here with a better explanation, but I thought I would get the ball rolling.
I see that Fede answered while I was writing this. Perhaps he can tell us how one identifies these as being for the MG15, and whether or not the MG34 mags used them also, and if they did, how one tells them apart. All three of mine had differences. One was made of aluminum.