I found this at a flea market this morning.
Headstamp is: MEN 70 .30 CARB.
It is made from a once-fired case (extractor makrs visible) and empty GMCS bullet jacket. The primer has been replaced with a dummy primer to look make the round unfired. The clip is also brass and is soldered onto the case. The whole round has also been polished and coated with clear lacquer.
It came in a small black false-leather box in the same style that cheap jewellery would be sold in.
I know it is unlikely, but could this be an item supplied to MEN factory employees?
Sorry for the poor quality photo.