What does this F indicate

[Tommo]

C always meant to me a Commercial truck motor to me.
I wasn't aware that they stamped this on the mag platform.
I'm learning facts as well here. 1930 HD cover close up0001.jpg1930 HD parts book inner cover0001.jpg
The C is explained on the inside cover of the HD parts book.
My 1929 Parts Book doesn't have a cover so I've copied a 1930 inner cover and it includes the V series in the explanation