I think the CS was when they started with the cross bolt saftey and the C models also have that but the difference now is C is walnut wood ... over the hardwood of the others.. but don't hold me to that..
I have a 1994 336CS and my owners manual calls it a 336CS which says the following "Your rifle is equipped with two safety devices, which when used properly provide an added degree of safety" which I take to mean it is a standard 336C with an added safety. The cross safety I believe was added somewhere around this time. Thats what I read in to their description of the rifle.
The 1992 Model 336CS I bought has walnut and the crossbolt safety, as does the current 336C. Mine does not have checkering, but I think I've seen older C models that don't either. Doesn't the reference to two safeties refer to the crossbolt safety and the half-cocked hammer position? - which all have.
The 336C was made from 1948-1983 the Model 336 CS started in 1984. The C stood for Carbine and in fact the earlier ones had the word carbine on them which was then shortened to C in 1948. 1951-52 PG stock with grip cap and white spacer. 53 35 Rem cal added, grip cap removed. 1956 drilled and tapped for scope and Micro Groove rifling. 57 shows a picture with the grip cap and spacer back. 1959 Gold trigger added. 1960 top and bottom of receiver sandblasted to reduce glare. 1964 32 Special dropped. 1970 100th Anniversary medalion put in the butt stock. 1971 hood removed and ramp added for front sight. Marlin/Osborne folding leaf rear sight standard. 73 sight hood added to front ramp sight. 1974 Marlin started heating the receiver tang end to fit the stock and had better wood to metal fit. 1982 gold triggers back to blue. 1984 cross bolt safety added and the model name was changed to 336 SC. 375 Added to the line and uncle Mikes hammer spur introduced. 1987 375 dropped from the line.