Factory Experimental: 1962 thru 1966. Think of F/X as the ultimate Stock category. Essentially for current model year vehicles with equipment not available on assembly line produced Super Stocks. Lots of confusion regarding F/X. Radically altered wheelbases and fuel injection were never allowed in NHRA Factory Experimental. Many legal F/X cars were eventually turned into AWBs and the car owners did not always remove the painted on F/X designation from the cars. The original Chrysler AWBs were intended to run in F/X. NHRA officials had other ideas. Their response was an emphatic “ no f---ing way!”
Experimental Stock: 1966 thru 1968. Five NHRA classes, A/XS through E/XS. These classes were designed for Funny Cars. You would have seen AWBs and the original floppers with this class designation. Classes A & B could use any fuel. Classes C,D,& E restricted to gasoline only.
S/XS: I see the S/XS class appearing in 1967 as one of the Experimental Stock classes.
The S stands for Supercharged. Classes A thru D were for non-supercharged cars. S/XS could use nitro.