There are four distinct slides on the 40P. The first is the milled out area with CZ-40P stamped inside. The second is not milled out, but slab-sided, with CZ-40P stamped on it. The third is milled but with the original CZ-40B stamp in it (mine). Presumably, this was originally stamped Colt Z-40. The fourth is slab-sided with CZ-40B stamped on it.
The CZ-40B has two types of slide, the slab-sided and the milled, both with CZ-40B stamped on it. My guess is a large number of the milled slides were originally Colt stamped and then milled for the 40B production. Some were never stamped Colt and so were "virgin" when they were built as 40B's. Some were still "virgin" when built as 40P's while others were milled down.
Davis