Early mags are known as a P15. Proper up to SN# range 304-70000. Newer mag is known as a P18. Proper for everything past SN# 304-70000.
The mag catch hole on the P18 is .040" lower on the body which allows the mag to seat "deeper" into the mag well to allow for more natural inline feeding into the chamber. People have modded the older [P15] mags to work/interchange with the newer [P18] mags, but I have always just preferred to buy the proper mag since they are readily available.
I actually figured out how to modify an older P89 to accept the newer mags... it only required filing off just a small amount, probably that .040", from the top of the magazine internal latch..... and the older mags will still work too with this.