Hi fattyre,
That was an original Raleigh fitment for many years.
There's a thick steel lug brazed onto the steerer headtube with 3 holes in the underside, turning the key raises a pin into any one of these holes enabling you to lock the steering in the left, right or straight ahead positions.
Unfortunately I haven't found the correct key yet so can't currently use this feature. I'm always hunting for one though.
I believe they were available on Raleigh, Rudge and Humber models.