What is a DB 9 to 3.5mm quad plug cable used for?
I finally found them. The cable is a firmware upload cable that came with our IOGEAR GCS1644 KVMs.
We've had them lying around too. They're usually thrown in the box to give you serial console access to a wide variety of gadgets. Have a look for ISDN terminals, video projectors or UPS devices. Could even be a graphing calculator!
In some cases, it is a specially made Video Cable for Polaroid®'s PDC-300, Fun! 320, & Fun! Flash 640 cameras.
From the depths of my grey mind I think there's a possibility that it's something used on a VERY old HP MSA SAN array (maybe an MSA 1000?). Happy to be wrong but I'm sure I've seen one years ago like that.