Z650 B1 vs B2 front forks

Yes, that's the same observation I've done from other parts catalogs. There are many things that can differ enough to make them incompatible, like total length, thread pitch/diameter/length and shape/size where the damper rod passes through, for instance. On the drawing, you can for instance see that there is a spring retainer at the top of the B1 fork, but in the B2 it sits below the spring.