It's essentially a 2" M15 that was made in the early 60s for the USAF. The contract was for 15,000 revolvers - S/N K500001 through K515001. The government destroyed all but a very few. Those that escaped death are very desirable and command $$$$$. I am not sure of all the differences except the barrel rib, rear sight base and tang are smooth instead of serrated. I think the backstrap is also stamped "US."