In 1960 Colt shipped the first .38AMU pistols to the Army Marksmanship Unit. There were 30 in this order. All were on .38 Super serial#d frames. These were the only complete pistols shipped to the AMU. All subsequent AMU pistols were "Kits" shipped without frames. There have been other shipments of complete pistols, but all are after 1961 till about mid 1962. My information is that there were about 100 .38 AMU pistols produced. I would think that your's is a "Kit" and was added to the frame.