It's kinda like Special K, only different.

The Series letters were just a way for Savage to keep track of small parts design changes should the rifle ever come back for warranty work so they'd know which version of a part to use as a replacement. There's no record of what the actual changes were for each specific Series letter so today it pretty much means nothing. The only exception would be the mention of Series J, which is usually improperly used to designate an short-action receiver that has the Gen I screw spacing.