I thougt I would share an information I received from Kawasaki about my bike. I made an enquiry about any possible information about this bike some time ago and today I received their reply.
Thank you for your message and your preference for Kawasaki Product.
However your bike is very old, then we have attached information only.
(See attached file: 1982 Z750-S1.doc)
Based on frame no, your bike is 380th of Z750-S1 1982 model.
Z750-S1 had been manufactured less than 1400 units.
Would you please check the detail looking at the above document.
End of quote.
So it is S1 model, but probably the front forks were replaced, because S1 should have only one front disc.