Has anyone ever seen or used one of these? Something which looks suspiciouly like one has come into my possesion, but its got no makers name, FCC ID, or anything...

Do these exist, or is what I've got some some of weird transponder from an ISDN modem or something? its got a female DB-9 connecter and a female RJ-45 socket on it

More likely it's just an adapter that converts the plugs and is meant for dumb terminals or printers or whatever to be used over existing CAT cabling for example.

Does it have any LEDs on it? Markings? Power supply?

No LED's, no markings, nothing. It 5CM of plastic casing with about 6CM long metal twisty things to attach it to the socket (they must have a name, no idea what though). I'll get a photo of it

It seems pretty recent though, came with a batch of old Dell Latitudes (P2-233 through 366 "old")

I have seen a usb to Ethernet adapter. I suppose it shouldn't be a suprise to see such adapters around.

Such an adapter would be rather pointless. Serial port max speeds are 230400 baud with special chipsets. 200 kbit serial port versus 10mbit ethernet is a too big difference. Most networking apps would time out when talking to a device that slow.

Serial to Ethernet Adapter

Does the unit look like anything from www.sanpeople.com?

-sanpeople.com