...Kawasaki used a lot of different style petcocks over the years, they look similar but there are internal differences so these "rebuild" kits out there are hit or miss - mostly miss 
That's my point. A petcock is a petcock is a petcock. Whatever the design, the purpose is the same. To have "on", "reserve" or prime and be vacuum operated. Even if a 1000J or a KZ750L uses two different p/n, the diaphragm is opened by the vacuum of one BS34 carb and feed four of those. I can't imagine any reason why diaphram would be different in those two petcocks, particularly that they are of the same exact design. I do have a spare petcock one here that I would like to rebuild but I ignore which precise bike it comes from so I'm clueless on which rebuild parts to order.
Maybe the design changed slightly over the years but the older part should become obselete and replaced by the most recent one. Anyway, I suppose we have no choice that live with this...