Curious ex-BT modems

Merry Christmas to all! Obligatory pre-feast Christmas posting... Hopefully there's some UK telecoms people on the list who can tell me some more about these. I unearthed a pair of BT modems (look to be from the mid 1980s), badged as Datel 4961x's. They're in nice finned metal cases (with plastic end caps; Datel unicase 4001X's) - I think I hung onto them intending on using the shells to house a pair of amplifiers, having never had any luck getting the modems to work. I was about to scrap them, and opened them up to strip and useful parts first. Now I'm curious. There's a *lot* of silicon inside these things. Most of the larger chips appear to be custom, badged in a range from MS2006 to MS2011. One of the 40 pin chips carries the marking "BTRL / 403 / 5113". British Telecom Research Labs maybe? There are three OKI 40 pin chips too, several ROMs, and no less than 42 DIP switches internally. There are six fuses inside - they didn't hold back there. The rear of the case has a 25 pin DTE socket, a telephone pass-through socket, PSTN socket for connection to the network - and also a mysterious "PC" socket (same type as used for the phone network connection). Any idea what that's for? ("Private Circuit" maybe) The front has push-switches for 4800 baud operation, plus AL, ST, RDL and DL. I assume ST is self-test, but I don't know what the others are. There are status LEDs for 4800 operation, DSR, RFS, TD, CD, RD and test. Those all make sense. I remember I never got one of these to talk to a remote modem of any type. I'm wondering if they're supposed to be used in pairs (or talk to some specific remote equipment that I don't have) and use some sort of non-standard data compression, or even encryption. Any ideas anyone? Anybody know of any UK telecoms lists where there may be people who can help? cheers Jules