Look on the steering head for the frame manufacture date and letter. the twin engine model usually came with a B as in KZ750BXXXXXXXXX. XXX´s represent the serial number. Alot of people also switched engines between the fours and the twos it was easy and therefore common for lots of reasons, so it could be that you got an 80 frame with a 79 or older engine. The 750B was only made from 76 to 79 though so find out and let me know what the back story is I enjoy hearing those kinds of stories.