That "J" was just a series indication used back in the day (1970's). Totally different than the context being discussed in the article linked to above.

Basically it denoted a small changed to one or more small parts on the rifle and was only used by the factory if/when the rifle would ever come back in for warranty work. Unfortunately there's no known records of what the changes were that each series letter corresponded to. In some cases the change was as insignificant as a supplier change for a certain part.

In the 2000's Savage started using the "J" in the alphabet soup to denote laminate stocks in the Royal Jacaranda color pattern. An example would be the old Mark II BJVSS (laminate, jacaranda, varmint, stainless steel).