I believe the rear fender/seat support located at the rear of the frame of dohc cb750's and 900's are the same for k and c models. F I believe is different. If you don't get this figured out before I have a chance to check my 750k, 750c and 900f. Will Get back to you if no one else has input.
1982 cb900F (2016-Present)
1979 cb750k (2010-Present)