As far as my 370/12 is concerned, the mods are almost as old as the 12-string itself (ca. 1970).  As you look at the pics of my 370/12, remember that the controls are basically the same as on a Strat; the top knob is volume, and the two lower knobs are tone.  Pickup selection is a 3-position Fender switch, and the little toggle is an in-out phase switch to get 2-pickup combinations like on later Strats.  The Strat switch is a bit unorthodox in terms of a Rickenbacker.  This guitar (SN: HJ1576) was not sold through a retail outlet but through the Rickenbacker sales office, in late 1968, to a music-related school in Oregon or Washington, and was a 360/12 at the time it left the factory.  About a year later, a third pickup was added and the stock knob configuration changed (maybe as a project at this school?).  The middle pickup has the same amount of wear as the other two.  Even though it looks a bit wacky, it can provide pickup combinations that a standard Rickenbacker 12-string cannot.  This may have been why it was such a popular studio guitar in Nashville for over 25 years.   -- John Simmons