full length sizing die and factory new cartridge cases

[browningautorifle]

care to explain the RCBS die letter system? A, B, so forth

Yes sir... RCBS stamped the top of their dies with a letter beginning in "A" in 1949 and ending with "P" in 1965. There after that used the last two digits of the year, 66, 67, 67 and so forth. I have not bought any new RCBS dies lately but all I have are dated either with a letter or two digits.

do they have a 357 magnum ONLY carbide die set

Not as far as I've ever seen. Even for .357 Maximum I would expect to use a .38 Spl/.357 Mag die set. I only use Carbide for pistol calibers now of course.