Would love to see pictures as well. If I recall correctly, there a couple M91's floating around out there with supposed U-Boat markings. Here's a thread from another forum:
Also, from: http://www.mosinnagant.net/ussr/Mosi...etsFromJPS.asp
"Rarer still, is another example of the captured Russian rifles, which was issued to the German U-boat, U 73. At the beginning of the war, each U-boat carried two Maxim machine guns for use by landing parties and for shooting and detonating floating mines. In 1915, when material shortages of every type were beginning to appear, the Maxims were recalled for issue to the army and in their place, each U-boat was issued two captured M91 Three-line rifles. The “U 73” marking was stamped into the stock just below the trigger guard."