George,
Yes your right in one respect. The first Fox 15 was the Rocket 15 and had a straight plug and steel fin cylinder and was really geared towards sport and lower powered stunt.
The 15X was a completely different design with a drop in liner and of course the different head with the slanted plug. It was a much more aggressivly timed motor and was probably geared more towards 15 sized combat planes .
Really the X was much faster then the steel fin, it also was a little more long lived.
Thanks Dennis