Welcome to the forum!
The 420 is a pretty cool cab. I've got one and am adapting it for a Hammond without doing any modifications to the amp. Yours should have a 6-pin connector, wired differently from the Leslie connections you'll usually find. Luckily, you can still use it with a Leslie (147 style) if you make a jumper and don't need switching. Switching in these doesn't control the motor like on a Leslie, rather it controls the speaker connections. The Spectratone has a solenoid on top that can be wired into the crossover to give you an interesting effect, I believe it was meant to emulate a Leslie without having a lower baffle. There are 3 relays inside the 420s amp, these control whether that solenoid is in effect, and which speakers get what. It's interesting, and a bit complicated
I have a diagram for making the jumper and motor switching, just like a Leslie 147-style single-speed. I'm going to build a separate box for these things, and mount it somewhere in the cabinet.