I recently purchased one. It has an unusual serial number on the guns frame vs the normal period correct location, on the front of the grip frame. I was fortunate enough to know Bob Rayburn. He was probably the nation's most preeminent Woodsman collector. I was going to reach out to him but found out of his passing, earlier this year on the Colt Forum. I contacted the poster, his close friend and premier Woodsman gunsmith Tom Platt. He is a great man and educated me on my Woodsman Sport but could not definitely identify this gun as experimental or a prototype. He did also reach out to another notable Woodsman collector without success. I did make the comment to Tom, that perhaps this was a "lunchbox gun". He agreed that the thought had occurred to him, as well. I am hoping there may be a forum member here, that has some Colt factory history, or expertise. Last night I submitted an archive letter request as well.
Have you taken it apart? Normally sn is on front of grip frame, inside of slide, and side of mainspring housing. Is x27 or another # on any of those parts? Nice pistola. 
