David and Puffs -
"Sidewinder" was a small sidecar attachment, enabling a learner rider to ride legally a larger capacity machine than the UK learner laws of the time would allow for a solo machine.
http://reddevilmotors.blogspot.com/2016/02/sidewinder-learner-sidecar.html
On another point : in six decades of buying and riding motorcycles, I have never had a test ride........
Jay Bee.