A friend set me this pic of an adapter a family member gave him. My guess is it was part of a military kit. Any ideas?
Thanks!

He says there are no markings at all.
How long is the case from rim to "OEM" mouth. If 51mm, then it's a 7.62 NATO. If 63mm, it is likely 30-06, and might be part of a Pedersen kit. Others will no more about those.
I will get him to measure it. The story he got was to just throw it in a 30-06 and go to town. So I assume it is a 30 cal.
Found it!
It's a cartridge holder used in the Hoffer-Thompson .22 caliber version of the
Model 1903 Springfield rifle. These were basically standard M1903
rifles, but with a .22 caliber bore. For the most benefit as training
rifles for indoor gallery practice, they used these cartridge holders so
the soldiers would be able to load the magazine from a stripper clip,
and everything would be done exactly as with the service rifle and
cartridge.
The spring and plunger/firing pin would be pushed back
and a .22 short rimfire cartridge inserted into the slot in the holder,
and then the holders would be loaded in the stripper clip and
everything was ready for use. There was a special tool for ejecting the
fired .22 short cases (basically a short rod with a wooden handle).
Supposedly they are worth about $35.