Andy Hall wrote:

They were called a Pedoscope and it was part of the ritual in any
Clark's approved supplier of kid's shoes.

[ ... ]

I reckon that they had disappeared by the early sixties.

Still in use Ooop Noorth (Wilmslow) in the later sixties - had my feet
done more than once with this brilliant (?) invention when a nipper,
can't have been earlier than 1967...

Stefek