...test article or prototype.....
Just about everyone in here is going to be more knowledgeable than me, but nobody thinks it might have been a test article or prototype?
Send off for the letter. It might tell you a lot about that s/n.
Did the OP say if it was alloy or steel? The frame that is.
Holy Cow, Run, Run from that as fast as you can. A" reasonable man" could observe the milled " slot" where the serial number used to be. Bubba must have stamped the made up serial number, IMHO.
I remember many LEO friends issued M-39's back in 70's. My first impression( no pun intended) was it Could Be A Stolen LEO issue 39.