The 6 volts were all 2-wire, and used an O/D relay in concert with them. The 12 volt solenoids of '56 & '57 were 2-wire as well(again, with a relay). Starting in '58, they did away with the OD relay and that required adding a third wire to the solenoids as well as modifying the internals of the solenoids some. BUT.... for '64 they went BACK TO a 2-wire soleniod, but it's not the SAME 2-wire solenoid as was used in '56 & '57. On the '64 models (part-way thru 8E production) there was a new 2-wire governor incorporated. So, you have '58 - on were 3 wire solenoids W/O OD an O/D relay. Still used a single-wire governor and a kick-down switch on the throttle, as had been the case prior to '58. Then for '64, they reverted to a 2-wire solenoid again WITH a two-wire governor as well.
BTW, the relays - while they function the same - are different for 6 or 12 volts.