sti6yr - Very curious. The SN, and the Model 10-5 designation (is that in fact stamped in the yoke cutout?) definitely date the gun to 1971-72. The SN range that year was from D330001 to D420000, and the 10-5 was manufactured from 1962 to 1977, when it was replaced by the 10-7 (the 10-6 was the heavy barrel version). In fact, if it has "Mod 10" with or without a dash number stamped anywhere, the gun can't be as old as 1953 because model numbering wasn't introduced until 1957. On the other hand, the half-moon front sight was discontinued in 1952, so would not have been original on a 10-5, on indeed any Model 10.
The only thing I can think of is that the gun has had the original ramped sight replaced with the half moon, or perhaps the entire barrel has been replaced with an older (pre-'52) one. It may also be that the gun was ordered with the half-moon front sight, but I don't know if that was possible in the early '70s.
Just out of curiosity, where are you that you have to register individual guns and get to see previous registrations?