The connectors are the same, one 8 way and one 5 way, making a 40 key matrix. It adds extra keys using a special 4 layer membrane which actually presses two keys at a time, so there is no difference at the keyboard connectors between pressing Break or pressing Caps Shift and Space.
I built a prototype replacement using tact switches for each individual key in a 5x13 matrix. This proved the principle, but wouldn't be practical.
I did design a PCB, but it would have been quite a pain to assemble each one with 58 individual switches, it would also work out quite expensive to manufacture, even as a single sided board
So now I have a new solution. I have designed a replacement membrane for the Spectrum+. This features a 5x13 matrix with each key individually mapped and a single 18 way connector
This fits in the case as an exact replacement for the original spectrum plus one.
Interesting to note I placed the connector to line up with a small cut out in the backplate and insulating layer which is exactly the right size for an 18 way connector. Was this part of Sir Clive's plan all along?
There is a piezo buzzer on the controller which beeps when the mode is changed. I've also set it to make a click sound each time a key is pressed. This is what the original Spectrum used to do, and it is a neat idea, so I
So there it is, the new fully remappable ZX Spectrum+ USB keyboard. To get a batch of membranes made will be quite expensive, so I would need to get a few preorders in to justify the cost. Please let me know if you are interested.
Update: These membranes are now available.