I have a marlin 30-30 made in 1957 according to the serial number.

My question is on the barrel it says (Model 336R.C.-30-30 CAL) what does the R.C. mean?

Model 336 Carbine (R.C. Regular Carbine)
This model was introduced in 1948 and was an improved version of the Model 36. It features a new type round bolt, chrome plated with improved extractor and resesigned cartridge carrier that improved feeding. It was chambered for the 30-30 WCF and .32 Special cartridges and has a 20" round tapered barrel with Ballard type rifling. The finish is blued, with the receiver top matted to reduce reflections. The pistol grip stock and semi-beavertail forend are of American walnut. It features Rocky Mountain-style rear and bead front sights, and the hammer is lowered to facilitate scope mounting.