According to Colt records serial numbers 210387 thru 212516 were reserved for receivers for spare parts. Serial numbers 212517 thru 215386 were reserved numbers and probably not made.
The Ordnance Department ordered spare receivers to be used as needed, and those that were not used were later built into pistols by Springfield Armory from spare parts. The phosphate finish indicates that it went back through rebuild some time after it's original build date as the original finish would have been blue.
It is a Model 1911 with the receiver being built in 1917. You indicated that it was a 1911/A1, so does the slide have the 1911A1 designation on the right side? It is one of the oddities of 1911 collecting and has some additional value, but not as much as if in original blue finish.