If you look at it, you'll see that the coil section (actually fairly small) can be unscrewed from a "base" section, which in turn is mounted to the chassis. I "believe" it's that base unit that you are referring to, or sometimes it's called the "power stage".

BTW - I was having some starting problems a couple years ago and just for the fun of it I separated the coil from the amplifier and found two or three electrical connections between the two units that are a type of "pin in spring-loaded receiver" affair. The contacts were quite gunked-up and not very tight fitting, so I cleaned them good and tweaked the receiver parts a little to make them fit tighter. I know my description doesn't sound like it makes much sense, but if you got into it, you would see what I mean.