Having just moved into a house with a built-in junction box (type JB 116), I was curious to see what was inside.
To open the box, rotate the catch 90 degrees anticlockwise using a pair of pliers. The lid of the box clips in at the top, so you'll need to lever it open from the bottom.
Here's what I found inside ours — mostly brick dust and cobwebs. It's probably not been opened in the last twenty years. What you can't see very clearly here is that our installation had cables going to two selector switches originally; one disappears to the right of the box and appears out of the hole visible in the previous picture (or would do if it hadn't been clipped and pushed back into the hole), and the other (which is no longer present) went out the back of the box straight into the lounge. If you grab the rotting piece of plywood, pull it out and dust off the masonry debris, things become a bit clearer:
The trunk cable enters top left and leaves top right (or vice versa). It's got seven pairs in it, one of which passes straight through, and the other six of which are connected to tap modules with screw terminals. The six tap modules are held in place by a couple of L-shaped pieces of plastic; each has two sets of pins for subscriber cables. One of the subscriber cables is bottom left, with the ends clipped off; presumably it would have originally had crimped pin connectors to match the pins on the tap modules. The other would have been bottom right, but I think that must have been pulled through from the inside after it was disconnected; note the broken cable clip and board.
The board was clearly hand-assembled, and the whole arrangement looks significantly more complex than the M117 boxes used in Hull. The labels on the tap modules appear to read "2-18, 2-18, 2-0h, 2-0h, 2-18, 2-18". (Perhaps the two different modules are for the radio channels?) The modules themselves are neat little bits of engineering: they're actually a little passive circuit with some capacitors, resistors and inductors potted in epoxy:
While I think it's unlikely that the system will ever be resurrected, please be careful putting the box back together, and be sure to close the catch again (else it's rather easy to drop the fairly heavy cover on your foot).