So I *think* this is an Atari cable, but not sure. One end appears to be an SIO passthrough perhaps, but what would the other end have gone to? Seems a lot like a cassette interface but it's larger than that on a Commodore Datasette.
It might be for a GE Compu-mate tape recorder which had a second box that converted Commodore or Atari signals for their seemingly standard recorder. The Compu-mate has a similar SIO to very wide connector that goes to the interface box. That might be an updated cable. The reviews and the few pictures I can find are for cables without pass through.
Ahh that does look very similar. Too bad I don't the Compu-Mate. Not that that's anything I'd actually want to use - kind of a convoluted looking mess.