Again this is just my understanding,
The early 77 bikes had spoked wheels and the latter ones had the comstars, you cant put a stock spoked wheel where a comstar was.
The physical difference other is the caliper mounts. the early ones had the bolts spaced far apart, the latter ones use the same spacing as the 77/78 CB750 for the calipers
I think it comes down to the fact that the spoked wheels are 6 bolt rotors and the comstars are 5, also the rotors have different spacing
I was looking into using a latter GL fork so i could run the OEM twin piston calipers but i wanted to run my lester wheels.
since this is for my K bike i have a lester that was meant for spoke wheels (6 bolt rotor) so i need to use the early GL forks to have everything just bolt together.