via this auction
"This is a vintage Yamaha model SK-20 analog synthesizer/keyboard made in the late 1970s. All of the functions have been tested. There is some static when the volume slide switch for organ, string and poly-synth are moved but the switches work fine. Also, one slide switch button has been replaced (see picture). The optional accessories, including head phones, foot switch and the foot controller pedal are not included. The cabinet has several dings and damaged edges and there is light corrosion on some of the metal fittings. The serial number is illegible. Included is an Ace padded gig bag that is worn and has tears on the corner, but overall it is in serviceable condition."