Boo...
It may be folklore...I don't know. But, what I was told by a buddy who used to appear on the Grand Ol' Opry was --- A lot of self-taught guitar pickers would wander into Nashville looking for their big break. Most could not read, but had great licks.

In order to play in transposed keys, some would use a Kay-Po (I'm sure that is not spelled right) and all they cared about was the finger positions on the fret board. Calling out chord changes was no help to them. So they developed a numbering system that worked in any key.

Maybe folklore...maybe not. But, that's how I heard it.

Eddie