From what I can figure there were 2 ways primarily that USAF model 15’s made it to the civilian market:
1) at one time, officers could purchase an issue gun from the government. Long before the model 15 era, it was at any officer rank. By the 60,s it would I believe be limited to generals.
2) when the 15’s were declared obsolete or surplus, police agencies could acquire them for department use for free. Part of the deal was supposed to be they would be returned to the feds when they were no longer needed. However as was mentioned sometimes these guns changed hands in police service with multiple agencies involved, and the idea of returning to the feds got lost in the shuffle. Also let’s say a department had 300 15’s, 250 were regular commercial guns and 50 were old Air Force guns, and when they transitioned to new guns the old 15’s were sold as a lot, again ignoring the return to the feds requirement

Also as mentioned, sadly when these were declared obsolete ( with the mass transition to 9mm pistols) they were simply destroyed- cut up crushed or melted