Non German guns used by Germans in WWII

The Germans used weapons captured from the various countries they conquered to supplement their own weapons supply.
I just obtained a prewar French Mas 36 and stamped in small letters just under the serial number in the stock is a small stamp, LK5.
The theory is that this is either a German capture & check mark or possibly a German unit assignment stamp. However there are apparently no German documents known so far to prove this theory.
A number of rifles from various countries which the germans fought or took over have been observed having a similar stamp there, either having a LK5 stamp, or other numbers after the LK. The French ones know with that stamp, all have the LK5. There is even a British Enfield having this stamp.

In an attempt to find more guns having this stamp to help add weight to the theory, could folks with prewar or WWII period guns other then German made ones, look to see if that stamp is present on their gun. The stamp is quite small and could be missed easy. It is usually located on the butt stock near the butt plate. Thanks for any help. Ray

Friend gave me a "Bringback" his father captured during final stages of the bulge , it was taken from German officer who had it as a boot pistol, it is a 1927 Spanish SEAM (Eibar)6.35mm Vest pocket auto pistol and it was thought to have been carried by said officer since Spanish Civil war duty? The tiny pistol has lotsa cryptic proof markings & stampings and I'm thinking it could have the marking you desire info on ?:cheers:

I am more than likely wrong on this but I was under the impression that the LK5 stamp signified "Vichy" provenance.

I believe the "Vichy" provenance was also one theory but because rifles from other nations the Germans fought or occupied also have those LK stamps. It was thought to be most likely a German capture, proof, or use stamp. As I mentioned, there has been no German documentation found so far to explain what the stamp actually represented, at least that I am aware of, Ray