For the slant plug Fox .15 there was four versions. The .15, .15X, .15XX, and the most rare of all the .15XXX. There are very few known examples remaining of the XXX. The XXX was a vry hot engine for what it was.
People degrade the slant plug Fox .15 but I had a lot of fun in the mid '60s with a .15X in planes like the Ringmaster Jr., Circus Prince, Berkeley Daze, etc..
BIG Bear
RNMM/AMM