Actually it is the distance from the top of the magazine to the hole.... at least with the two I have.
Old style mag's is 1.68
New style's is 1.7
So the difference is actually where the magazine lock pin is located on the two pistols... I fixed an older P89 by taking the pin out (this is a real pain) and filing about 2-3 hundreds of an inch off the top.
Oh, and the older mags will work in the newer pistols anyway. they just extend out of the mag well a tad.