I also noticed several other differences betwen the aftermarket "black" and "green" seal. First off neither has the steel shoulder built into the upper part of the OEM seal. This shoulder provides stability and reduces distortion of the seal lip. Speaking of seal lips....the OEM seal has a very distinct lip....the other 2 do not. A big difference is the actual ID of the stem hole. The "black" seal hole is noticably larger than OEM. The "green" hole much, much smaller.
I just did a head for Branden and he supplied a set of both the "black" and "green" seals. I did not put them in for all the reasons I have listed.
I have not seen the Kibblewhite seals for the 750 though I will order a set to check them out. The Kibblewhite 550 seals are good IMO for all the reasons I have spoken about.
I know it sounds kinds stupid to go over something as simple as a seal BUT if you have invested a lot of time and money in a project the last thing you want to happen is have it blow smoke.