1979 Chevy Silverado What is this C10-20 & S10 -20 Silverado

I know this must be a unusualy question. I recently in herited a Pikup from my father. But it has very few lables and marking on it. I know its a 1979 Silverado with a 350cu. Single tank. with air. I have seen alot of differnt descriptions on line ie: ( C10, C20, S10, S20, 2 ton , 3/4 ton ) whats the differecne and how can i tell what i have? The grill has Silverado 10. notheing else and the fenders have nothing on them either. If need i can insert a pic.