If that is a >>FP<< on the left side of the rear lug, I wonder if Friedrich Peter of Judithstrae, 40 - Suhl applied those letters. He was active during this period? I see a few >>Boxed Ks<< noting the tube work by a Suhl K mechanic like Klett & others.

Great image of the menacing muzzle.

Cheers,

Raimey
rse