I compared Colt / NAC5 with original Colt guns on hand and concluded that it was an interesting variation. It is definitely an original Colt receiver being clearly marked. It was plain that it was not a complete Colt gun and was not likely to ever have been one.
In this respect I agree with Gordon. It is quite possible that none of the Colt/NAC guns now existing were complete when Numrich discovered them. More likely they were receivers and frames and parts. Also likely, every existing Colt/NAC gun is a bit different.
In the Cox book, page 24, he refers to 1928 models being sold to the Virginia State Police. Roger Cox states that these guns were originally Model 1928A1 which had the numbers ground off. No doubt it is possibly true. On the other hand it is obvious that Cox never saw a Colt/NAC gun or he would have surely made a comment about it since his book was primarily a Colt TSMG book.