Proutfoo said:
So I guess that my Nazi marked 870 Express is a fake

oh well.
Why would the germans bother to stamp captured enemy rifles anyway? Just curious.
There's an excellent writeup on this topic in Terrence Lapin's "Mosin Nagant Rifle".
Basically in WW1 and WW2, the Germans never managed to produce quite enough arms to equip all the soldiers of the Reich with the latest kit at the same time - battlefield losses were too high. As a result, even in WW2, rear echelon troops were, in some cases, using old Steyr M95's, a few Carcanos, some ww1 era Gew98 and Gew88, and also alot of captured French and Russian gear (and later American gear like M1 carbines).
Earlier in the war when things were still going well on the Eastern Front, captured rifles in good repair would immediately have been issued to rear guard troops, signal corps, etc. to free up much-needed K98k's for front line use. Especially since replecement rifles had to be trucked from Germany and frankly, that truck space was better used to send gas, food, ammo, etc.
Pre-1944, the WaA used to have inspectors at field armories aiding the weapons techs and inspecting repair work - this was before the manpower shortages of the late war period. Even foreign arms would have been stamped as inspected if they went through a unit armory or repair facility that was WaA staffed.
Lots of examples are out there of German marked pre-1940 MAS36 rifles, pre-1943 SVT's, and likely even Mosins.
During WW1 this practice was even more widespread, the bulk of captured WW1 era M91's went to Austria and turkey, but many also served the Deutschreich and have a large branding on the buttstock to prove it. Many of those same rifles later ended up in Finland after germany gave them to France as war reparations and France then sold them to Finland, etc.