Very early Combat Masters would have a "Pat Pend" on the left side. Only blue guns had this. (Low 4 digit serial numbers). Later blue guns with shiny sides on the slide would have a "MK VI" engraved under the "Detonics 45". If it was a MK I (which was a designation given after production had been going for quite a while) it would say "Combat Master" under the "Detonics 45" engraving on the left side. (True also for most of the stainless MK V's). As posted before, the headgunsmith would engrave only "Detonics 45" on the slide if he built it, or slicked it up, or replaced the top end. Later, that kind of "trade mark" was copied by other employees. Even later it appears (in the last year or so of production; during the re-organization efforts after the bankruptsy) that the Combat Master slides were simply engraved "Detonics 45" no matter what "MK" number they were. As such, most of the C2xxxx blue guns that I've seen on the used market simply have the "Detonics 45" engraving.
Hope this helps, ...Deeton