I'll go out on a limb here and say you have a long-shank barrel (but not military spec) fitted to a short-receiver. I can see the adapter ring between the barrel flange and the receiver.
The barrel profile does not meet the military blueprint specifications/dimensions, or if so, it has been 'massaged'.
Most likely an aftermarket barrel fitted to a military [Erfurt] receiver. Barrels wear out, and get replaced.
Looks heavily buffed. Probably barrel replaced at the same time.
It's also a Revisions Commission Erfurt.
Just my $.02...
