The GPZ 900R was initially produced with a 10mm upper mounting for the shock absorber, however a number of early models suffered from snapped bolts, so Kawasaki increased the size of the upper mounting from 10mm to 12mm.  All GPZ 900R A2->  feature a 12mm bolt.

Many aftermarket shock absorbers feature a CNC machined lower mounting bracket which is often wider than the one on the original unit. Consequently when swapping suspension units, be very careful to ensure that the lower mounting bolt is long enough to ensure a secure fitment.