How dull is the matte finish? Is it a satin sheen? Then it would be a MK V. If it's a real dull grey and somewhat rough, its likely an MC-1. You might look at some of the pictures on GunsAmerica.com to help you figure it out.
The rubberized mainspring housing on either would have been a genuine Pachmayr.
If, on the side of the slide that says DETONICS .45, there is no other engraving, then it's likely that the slide had been replaced. One of the smiths in the repair department had a habit of only engraving DETONICS .45 on slides that he replaced.
Sometimes a malfunctioning gun would be returned with the customer demanding a new one. Sometimes that request would be granted. The returned gun would then be made to "run like a swiss watch" by said smith, which may have required a new slide. In such cases, after test firing the gun, it would not have been unusual for the re-manufactured gun to be refinished and sold as new. Such specimens were VERY reliable Detonics .45's!