Auto Pistol Adapter

I picked up this auto pistol adapter from one of the germans in St. Louis last year. I cant identify.
It is larger diameter than 6.35/25 acp.
.308 rim
.227 cm
.745 ol
inside diameter
Adaptor came with a 4mm practice cartridge m20 that fit into adapter perfectly.
I seem to recall this was made for a specially chambered pistol to get around certain gun laws by having a odd adapter cartridge size.
What do I have and what pistol is it used in.
Bob R.

IIRC, 4mm adaptors like these were commonly available in most German pistols, to allow indoor target-practice (“zimmerstutzen”?); I’ve seen them mounted in Lugers, P-38s, and Walther PPs/PPKs, but .25 Auto is a new one on me. Besides the “cartridge”/adaptor, they also required a little barrel-liner that was held in the pistol by a knurled cap at the muzzle.

Bob - I think you have the right information. I don’t subscribe to DWJ (Deutsches Waffen Journal) anymore and haven’t for some years, but for awhile, the ads from some of the bigger dealers in Germany had all manner of pistols that had been converted to 4m/m