It looks like a regular C64 cartridge, with a serial cable added...
Brilliant! Certainly another "shut up and take my money" thing to add to my collection. :)
Thanks for the information! I recently got an SD-box--that's a device that allows connecting a micro SD card to a Commodore 64 and use it as if it was a 1541 disk drive, so I'm currently playing all the great 8-bit classics in their C64 versions. I hope they will make something similar for Spectrum.