At an auction today, a friend purchased a German-made, WWII-era Mauser K98. It appears to have all the appropriate German markings, plus ... the letters "USNR" stamped in large letters in the stock.
The previous owner was in the U.S. Navy and trained at the Sampson naval training station on Lake Seneca in New York.
Is is possible that this gun was war surplus/salvage and sent to Sampson for training use?
There is a very small proof mark on the receiver that might be a "flaming bomb," but it's hard to tell for sure.