A cursor emulating mouse kit from members of the Merseyside QL group in the 1980s.
Supplied in kit form, the interface was usually installed inside the QL with a socket to allow a mouse to be attached.
There were a few examples of these built into external cases, plugged into the CTL sockets too – the picture shows one I came across with a dot matrix printed label indicating ‘Mersey Mouse mk 2’ with a switch which allowed you to switch one of the mouse buttons between ENTER and ESC.
These scanned documents are from Steve of QBits and include a circuit diagram and installation instructions. Not that many of these were made, so if you get hold of one of these rare interfaces, you have a collector’s item on your hands! (Scanned as three JPEG files).