I've got a couple of those for 32acp in 30-06. Oldies from an estate sale some years back. Buried somewhere in the mini-cave also have a few more from the Mel Tappan era ( 22 in 223, 30 carbine in 308). Slow shootin' but fun on occasion.
Auxilliary Cartridges, Chamber Adapters, Cartridge Holders, Sub Caliber Cartridges, whatever you want to call them, have been around since the 19th century. For civilians, they gave the option of using a big game rifle for plinking or small game. For the military, they were a way to train troops without expending a large amount of full caliber ammunition. They were even used in artillery ammunition.
Collecting them can be a specialty on its own and some of the rarer ones can command high prices.