I agree that the first slide you show, above, does look as though a Colt roll-mark was machined off.
I had to copy the second image and put it in a photo editor to see what you were describing. That slide looks as though NO material was removed -- as the cocking serrations look unchanged, as do the depths of slide openings where pins are put in the slide. (If a complete layer of material was removed and the cocking serrations re-cut, the pins would have be be different (shorter) as well. If the other side was matched, the slide would be noticeably slimmer than other 40B slides.)
What does the other side of the slide look like on the the modified 40B slide, and the 40P slide? Did they cut the first slide to have a similar shape? (I think you said they matched both sides -- but mentioned FRAME, not SLIDE.) How about the 40P slide's opposite side?
The 40P looks as though it's a different animal altogether, with part of slide machined away in an area at the front of the slide where no material needed to be removed.
Perhaps CZ rejuvenated SOME slides (i.e., Z-40 and 40B) and maybe some additional ones were made, too -- for some additional 40Bs and for the 40P, and those additional ones were just roll-marked.
