Armourer - I have the exact same “round” except mine is in very bad condition. I was told when I got it, and also at separate times when I got two slightly different ones of aluminum, that they were the “filler” rounds for German 7.9 x 57mm drum magazines. Evidently the drum magazines are of such a design that were they to simply use live ammunition to the full capacity, the last two or three rounds will not feed, so these are built into the magazine, so that no live rounds are left in it.
I have no idea if this is correct, but other people who have seen mine said that they came out of drum magazines.
It has mystified me why they have to be more than one solid piece - that is, why the steel one has the moving part in it (I didn’t know it came out - mine is pretty much rusted in)
I have two similar items made out of aluminum. They have a steel pin through them, right below a deep groove around the circumfrence of the “cartridge” about midway up the “case”. I do not know the function of the pin, and gain, not being familiar with the design of the German drum magazines for various guns like the MG34, etc., I don’t know the function of either the groove, or the pin, anymore than I know why the steel one has a big hole in it with that separate part inside.
I hope someone more expert on the magazines themselves can enlighten both of us. If needed, I can post a scan of mine as well.