I have some of these. Usually they were used as printer extension cables or to a mechanical switch to share one printer with two computers or two printers with one computer.
Okay. At a first glance, I got excited and thought these were laplink cables. Another odd cord, 9 pin serial to a smaller version of a speaker cable. Any info?
That is an extension cable which can in most cases be used either for a parallel or serial port connection. Same cable, just make sure that both sets of ports that are connected to the extender are of the same type. Don't join a parallel port to a serial port.
Does the cable have any text written on it? Some similar looking cables can have wires that do not pass through completely on the same pins as one would expect.
While the Apple external SCSI uses the same 25pin connector, I would recommend not trying to use one of these extenders with external SCSI devices. Long cabling and often poorly built connectors led to signal loss which becomes much more noticeable with a SCSI hard disk.
You might want to provide a picture of that adapter. I can think of several different cables similar to what you describe including power adapters that leech off a serial port, controls for some forms of projectors, and some more exotic forms.
I actually have one of these cables too. I don't have a need for it, but I'd suspect it would be used to transfer data to and from portable devices (cameras, PDAs, GPSs).