Eeeeeeeee, when I were a lad ..............
You could ride any size bike on L plates if it had a sidecar attached. And the definition of sidecar was basically another wheel stuck out on one side. The craze round our way was for big Brit twins - Matchless/AJS 650's, BSA Gold Flashes, Enfield Connies etc, the ones at the lower end of the desirability spectrum.
Then you stuck a sidecar chassis on the side. No bodywork, just a flat piece of plywood about 3/4" thick with a bit of foam padding and a vinyl cover. This was known as 'the board'.
Then you took the 19" bike wheel off and fitted a scooter wheel.
Now you could play at being Chris Vincent,
the top British sidecar racer of the 60's. We take it in turns 'driving' or being 'on the board'.
The passenger would lean across the back seat of the bike on right-handers ...

and lean off the side of the board round left handers, to try and keep the chair wheel on the deck....

This didn't always work, and a mate of mine could get the chair wheel about a foot off the deck on a bend on Derby ringroad while the passenger (usually me) was hanging out into space. We passed a Panda car one day (Moggy 1000) like this and the cop never batted an eyelid. And this was in jeans, bumpers (cos they gripped well while you were moving about on the board) and no helmets.
Frankly, I'm amazed any of us made it into the 70's at all