The 6502 second processor was the first of Acorn's second processors, coming in the familiar cheese-wedge box which is exactly the same shape as the BBC Model B, except it is only half of the width.

Second processors plug into the Tube connector of the BBC via a 40-way ribbon cable, which can be seen here.

The cable itself is ludicrously short, allowing the cheese wedge to sit no further away than flush up to the right-hand side of the Beeb it is connected to. That's not always the most convenient place, but there's very little can be done about it!

Attempting to lengthen the cable by using a longer piece of ribbon cable doesn't work as it causes timing problems over the Tube interface, so the Beeb won't work properly. I have heard that this can be cured by using some buffer logic circuitry on the cable, but I've never tried this out.