Jorgy, I got to thinking, and there are indeed some ways you can tell the difference between some, if not all, K models. However, such detective work may require the engine to be open, and a lot of tedium comparing parts numbers online. In case you want to go that route, K1 parts are listed on this page:
https://www.cmsnl.com/honda-cb750k1-four-1970-1971-usa_model14354/partslist/ I sometimes go down that road myself.
Strictly as an example of something external that I can see, on my engine, that is different from K7 or K8 is that my K6 front sprocket is held on with 2 bolts and a fastener, whereas K7 only uses 1 bolt to hold it on. Ya can't just slap a K7 sprocket on a K6 and bolt it up.
Our aluminum engine covers, at least from K1 to K8 can be swapped easily, so we can't rely on them to tell us the actual year of the internal parts.
I have read that there were changes made to the crankcase in K1 and again in K2. I'm sorry that I don't know what they are, but somebody here will.