Yes, Harold. You are right. The letters are the first letter of the family name, so the source.
But, Hans Fay got an "F". Like this:
http://www.me109.info/display.php?a=e&fid=1632
Possible an other pilot was flying this plane and Fay fly it too.
On this machine we can see "HI", so its possible there was two man with a "H" as first letter in the family name.