MOOG Sound Producer

via these auctions where you'll also find Cynthcart and Prophet65 for the C64.

"Moog Song Producer (MIDI and Trigger interface) for the Commodore 64, with sequencing and MIDI-processing software. The Moog Song Producer boasts:
* 1 MIDI in
* 1 MIDI thru
* 4 (!) MIDI outs
* 8 drum trigger outs
* 2 Footswitch ins
* 1 Clock/sync in
* 1 Clock/sync out

That means the Song Producer’s capable of interfacing your MIDI and pre-MIDI gear. This item is in great condition and includes the original box. Just check out the photos. It include:
* Song Producer hardware interface
* Connection cable (it plugs into the Commodore 64’s expansion port)
* Ring-bound manual with software disk (and TESTED backup copy that I made)
* Original warranty card
* Technical service information leaflet (includes schematic, picture of circuit-board layout and complete parts-list)
* Quick-start/introductory guide (x2)

Perhaps most excitingly, though, is the possibility that---with the detailed technical information included---a competent C64 programmer could use it to control the C64’s SID chip through MIDI, as it just runs on a stock Commodore with a stock interface (the cartridge expansion port)."