Legal meanings, as found in 10,000 court decisions:
"XO class consists of overhead valve (OHV) and flathead inline and flathead V8 (except Ford & Mercury) and V-12 engines, 1959 or earlier design, up to 325 cid"
Permits Buick nailhead, Chrysler hemi, poly, B/RB, Cadillac, Ford Y & MEL, Oldsmobile Rocket, Pontiac, Packard, Studebaker OHV V8.
"XO class consists of overhead valve (OHV) and flathead inline, and flathead V8 (except Ford & Mercury) and V-12 engines, 1959 or earlier design, up to 325 cid"
Does not.