The 75 CB750F came with a #5 plate in the middle just like the K's had. That #6 was a 'field fix' for a noisy rattling clutch pack. It is a middle plate from the 75-79 GL1000. It takes up a little extra space. This requires a little mixing and matching to make work. To accomplish this you remove that #5 middle single metal plate and insert the #6 GL1000 double metal plate AFTER you remove the inner most #3 friction disc that is closest to #1 basket.
After Honda came out with the 'field fix' hybrid they did an update in 77/78 that has a few different parts including #1, the clutch basket which was slightly longer to accomodate ALL the discs without removing one for the 'field fix'. #2 was also slightly longer. The cover which is not pictured in your schematic was modified to provide slightly better clearance which you can do by grinding down those radial ribs slightly.
Here is the "field fix' from Honda. If your clutch is noisy you can do the field fix or get a whole clutch assembly from a 77/78 F or K and use the whole thing. That may be just as easy to find a complete unit vs trying to find that single double metal disc. I know
I used a 78K clutch assembly in my 75 750F with new Honda friction discs and HD Barnett springs. Works amazingly well. The 75 unit will have the field fix done and go into the 78 K engine when it gets built.
