Colt Armalite AR15 601 Very Low Serial Number

Colt Armalite AR15 601. Tried to letter gun from Colt, they will not letter Machine guns. My research makes me think the gun is first year production, more than likely the first Month. According to all the local experts, the gun is correct. It came from the City of Santa Fe in New Mexico in early 2000, where it had been in inventory since July of 1961 Am not an expert, but would be glad to try to answer all questions. Best Mike