OK.. out of curiosity.. I went to www.xtremeusa.com and compared part numbers on the fiches...
I didn't go back past 76 though..
It seems that the 76 K and F, the 77 K and F, and the 78 K models share common parts numbers for the ring sets.
The 78 F model shows a completely different number for the ring sets..
That doesn't mean that the pistons are all the same.. just the ring sets..

Build it the way you want it ..... then learn how to ride it like you own it..
www.lowridersbysummers.com