The replacement for the F4-750, when first shown, was announced as an F5.
The serial number range was registered with whatever international authorities that control such things as "F5".
When I moved my '07 F4-1000R from CA (where it was registered/titled as an "F4") to NV, the NV DMV said that according to "VINA" - the authority they use to verify VIN's - the bike was an "F5", and my NV title/registration is of an '07 F5.
AFAIK, there has never been an "F5" designation used by the factory since the "1000's" went into production. That your cases (and mine too - I've never looked) carry an "F5" mark is probably due to the fact that they just didn't want to change the molds or stamp.