The Mini set isn’t the only one featuring this Shuckle post stamp. The Ho-Oh sheet from 2000 features Shuckle as well:
Source:
And out of curiosity: do you own all or almost all Shuckle items you mentioned above, or you’re still looking for most of them?
Either way, amazing job cataloging all of it, and thanks for sharing! Some of those companies/years you’ve mentioned will also be useful when I’ll share my Seviper non-TCG collection. 
Greetz,
Quuador