If I remember right, Ludwig Loewe made those, from a large stash of 7.65x53 brass they had on hand. They were meant for use in the 7x57 chamber, but there were some problems (i.e., occasional blowups), suspected to have been due to the base of the steel-jacketed bullet upsetting in the unsupported area of the chamber, then fracturing at the cannelure with the base staying somewhere in the barrel. Predictably, the gun would blow on the next round fired.

I'm not sure I buy that explanation. I tried to duplicate the problem using original steel-jacketed bullets and short brass, but never had a bullet separate.