Casiotone MT-70. Not technically a synth, but check out the bar code reader and bar code music sheets. Title link takes you to more shot pulled via this auction.
Update via Loscha:
"Here is an article from "Your SPectrum" magazine which has info about interfacing a VL-5 to a ZX Spectrum computer via the barcode port. Like MIDI, well, only just sort of."
Update via .alex in the comments:
"It's sorta like my yamaha portasound pc-100 playcard system... but it reads cards with a magnetic stripe!!! i took some shots for you matrix. [link]"