Yep,
Parts bin special.
However BSA did make a tele fork conversion for WM20's.
It consisted of a pair of top & bottom plates that were machined off center to each other such that the holes in to bottom plate were a little further forward of the top plate thus positioning the front a little further forward.
This means that the forks do not turn in the same axis as the steering stem which in turn makes the steering fell really funny at low speeds.
AFAIK it was a dealer only part so they could buy up surplus WM20's civilianizes them & flog them off to joe public as "new" bikes.
I have come across a couple down here & a now deceased club member says they were fairly common for a short while after WW II before every one got enough money for one of those nice new swing arm jobbies.
Trevor, Ive heard this before & it seems strange to me that BSA would make a part that gave odd steering geometry, are you sure it wasn't just some skulduggery at one particular dealership ?
I just doesn't sound like a BSA produced part to me.