Right on the duplicate numbers in that range, so the slide is evidently a genuine Colt.
The reason I have nasty suspicions of the person who marked that slide with the "Colt 9m/m" is that (IMHO) it was done for the purpose of deception. If a gunsmith simply wanted to convert a M1911A1 to 9mm, he would not (or at least should not) have marked it "Colt" and tried to make it look like a factory job. Lots of stuff like that turns up in junk boxes at gun shows, and sells for a few bucks. Who knows but that that slide was the reject and a better one is on some advanced Colt collector's pride and joy pistol, a "rare" wartime 9mm "made for the OSS" or whatever.