It's chunky, grey and has two buttons.... 

Datel Electronics Genius Bus Mouse (GM-7) for the Sinclair Spectrum

 in it's original hard plastic box, both in absolutely perfect condition.

PLUS;

I can also happily include the original instructions! 

Specifications: 

Connector: Customer Assignable

Sensor: Optical Rotary Encoder

Speed: 500 MM/SEC

Resolution: 0.12 MM/DOT. 200 DPI

TTL Level Output