If you haven't already, check out "US Military Match and Marksmanship Automatic Pistols" by Bill Jenkins. There is a small section on military 41s and 46s. The book states that all branches of the military purchased 41s and most were stamped "US" on the left side of the frame following the serial number. I'm assuming the AMU guns would have been stamped this way. The Model 46 was developed for the Air Force and most were stamped "AFPG" for Air Force Professional Grade.