Unknown RS232 Interfaces
Here is a list of some "unknown" RS232 interfaces. "Unknown" meaning that the devices are so rare that nobody did ever see them. And it isn't even confirmed that they did exist at all.
Below adverts do sound as if the companies did actually develope their "own" RS232 solutions - although it's also possible that they only redistributed known RS232 interfaces from other developers.
If somebody can confirm that the hardware does really exist: Feel free to remove them from this "stub" page, and put the info on a separate page that describes the device.
Griesmayr Electronic advertised "E.B.S. 464" (Expansion Board System) modules for 464/664: a IF1 parallel/serial data interface (RS232, plus 8bit Printer Port). As well as IF2 universal I/O interface, and SE1 5-slot motherboard, and SK1/SK2 cables.
Gundermann Mikroelectronik - advertised a dual highspeed rs232, and (a separate product) a 48-pin parallel I/O interface.