I’m fixing this one for a friend. Introduced 1958. In this view we see the “pattern sequencer” — a large printed circuit board (the only one in the unit) with a set of contacts on a rotating arm. The contacts close circuits and generate trigger pulses, which hit vacuum-tube ringing filters to generate most of the drum sounds. This unit was in remarkably good condition, if you don’t count the ugly black padded naugahyde someone put all over the outer cabinet.