Europe got K0, K1, K2, K6 and K7. No K3, K4, K5, K8, F3, maybe no F0 either.
F bikes... I have only seen and heard about F1 and F2.
I have a K2 titled 1975 which look more as a US K1. The cases I use for the build is K2 engine and has marks to have used the single row ball bearing on the final shaft. Case can use the later double row ball bearing which it will get. Different shaft though.
My K2 will get the old style grab bars, later got one bar that goes all the way around the rear part of seat, fastened as the 2 single bars, shock upper bolt and rear fender/blinker bolt.
Most K6 here has title year 1977 as mine despite it was built Q3 -76 and got registration October 1976.
Important to see and know the differences of the engine internals. K6 has different clutch, outer fiber plate has wider tabs. Clutch basket look different and is fastened to the shaft thru primary hub with a clip and different spline washer. Later engines got deeper clutch hub and a double riveted metal disc. The clutch cover is different to handle the thicker package. I think that the only CB750 in Europe with that clutch is F2. Clutch fibers same as the K6 though.
The USA bikes got different rear lamp K2 and blinkers K5 according to CMSNL charts.
European K2, K6 bikes have the old blinkers and rear lamp as K1. This need the old type of rear fender. I found a NOS rear fender to my K6...bought as K1 with correct holes for rear lamp. My K6 had a mudflap fastened with 5 screws in rear fender. I have not seen those fenders as spare. K1 that fit has only 1 hole. I found some NOS mudflaps in France last year.
Hondaman has described the differences of timing, cams, carbs and hp. First CB750 had 67hp on crank, later 63hp. Some guys have written about 50 on rear wheel. F2 got over 70, maybe 73 hp on crank.
CMSNL spare part charts show some details but not correct rear fender stuff for my euro K6, it has US styke with big lamp and 3 additional holes on fender for the support of lamp.
https://www.cmsnl.com/honda-cb750four_model14344/There are more details to be aware of