What's This Stamp I Found In An Album?

This stamp was on a blank page of the Belgium section of an old album I bought. I looked on StampWorld at all section of other types of Belgian stamps and cannot find it anywhere.

I used Google Lens and it identified it as a Belgian stamp released the same date as pictured on the stamp, but today is identifying it as a German stamp of 3 pfennigs, although the stamp clearly identifies the denomination as Fr. (so much for AI).

Any idea what it is?

It's the lower half of a Belgian tax stamp of 1931
https://www.lastdodo.de/de/items/10561649-godin

It's the lower half of a Belgian tax stamp of 1931
https://www.lastdodo.de/de/items/10561649-godin

Thank you