Hhmmmm... I now confess to being stumped. I have had a look at the plate and it actually looks as if the colour scheme is right for CAT-5 UTP pairs. However, if you say that there are commoned connections in the connector, where each one is presented at the back as a seperate terminal, then it's maybe any of:

- shorts inside the connector itself - spring contacts bent together in the aperture? Solder blobs inside? Loose multi-strand inside the connector?

The only time I have seen shorted together terminals has been on these useless "Easy Wire" sockets, but the pattern is different to what you have buzzed out. It eliminates one useful pair, which doesn't make any sense.

OK, I know you stuck a cable in the front (known good) and buzzed between cable end and backplate, but have you done this ONLY on the backplate, ie. 1-2, 1-3, 1-4 etc. 2-1, 2-3, etc. and built a truth table? This way you eliminate the cable completely. I'm going to keep thinking about this, but I am stumped at the moment.