There's your answer.

I would trust Kuduae's knowledge and expertise on matters regarding European firearms and their history.

I wonder if the barrel threads were for a 'cutts' type compensator rather than a suppressor (silencer)? I'm just guessing, but here are a couple of examples from the 'net:

These are from a chat room thread on Gunbroker. The writer calls it "An integral muzzle brake at the muzzle, forged with the barrel..." of "a full stock Model 1952 Mannlicher- Schoenauer carbine in caliber .30-06, manufactured in 1953..." (I suspect it is soldered on rather than 'forged').

Here's a brake that has been added to a 98k Mauser, converted to 8X63:

Another odd speculation - could the previous owner have been using a blank firing adapter (BFA) such as these that were for Mausers?:

Thread protectors for same:

Cartridges loaded with wooden projectiles for above