[Bob Don]

ISCA (show car sanctioning body) has competition classes that break down very similarly with NHRA classes. It just could be these cars are lettered to fall into these classes?

From the ISCA rulebook:

"3330.0 Competition Stock — 1960 or later drag type stock
cars. Must have seat belts, may have a shoulder
harness. Rear tires can be no larger than 10.5” slick
or 12” street tire. Bolt on traction bars and ladder
bars are allowed. Wheelie bars are not permitted.
Multiple carburetors and factory styled fuel injection
systems are allowed. Must have full factory style
interior; no interior gutting allowed. Rear seat may be
removed for installation of roll cage or roll bar. Must
be carpeted.
3340.0 Super Stock — American factory produced drag type
high-performance cars. Roll bar or cage and shoulder
harness. Engine in stock location. Any size street or
racing tires. Must have full factory type upholstery;
rear seat may be removed for installation of cage
or roll bar; must be carpeted. Any intake manifold
permitted provided unit is same configuration as
original."