From what you wrote it sounds like this is an older P89 and takes the older magazines.... as I posted a year or so back I actually modified an older P89 to take the newer P magazines that would also work in the original PC Carbines... I think I took about 2,000 of an inch off of the magazine release part... not sure if it was the bottom part or top... might want to look at my post above it might be there....
Edit: I just looked at the two different magazines and the different in the latch hole on them is .042"
Here is a picture explaining the difference and like I said if you take that amount off of the top of the mag latch, the new magazines will work in older P89s.