Sporran maker?

[OC Richard]

The question is what was the black paste that hardened and gave old leather sporran cantles faces their shiny facade. It was clearly an applied application as it tends to warp, flack and run or time.

You're talking about what was called "Japanning"

https://en.wikipedia.org/wiki/Japanning

Japanning was also applied to leather, which could be called either Japanned leather or Patent leather

https://en.wikipedia.org/wiki/Patent_leather