The CR plugs are commercial grade and have a thicker electrode and ground. It's also a slightly hotter plug than the R.
Basically a heavy duty plug used much in GM small block TBI trucks and other GM vehicles. I think they used them in some Corvettes as well.
0.035 inches for CR43TS or R43TS AC Delco spark plugs
The 1974 Camaro Z28 with electronic ignition typically came equipped with AC Delco R43TS spark plugs. These plugs are designed to work effectively with the vehicle's 350 cubic inch V8 engine and electronic ignition system, ensuring optimal performance. Always refer to the owner's manual or a trusted automotive source for specific recommendations based on your engine's configuration.
That car didn't come with a 350. The V8 available for that Monte Carlo was the 305 which took the ACDelco R43TS spark plug.