@1steditionbs , From a technical/material standpoint the peel away CoroCoro promos would have to have been made with a glossy finish on the face as to be able to remove them from their contact sheet. Much like the way stickers peel off of their backing. If they were matte they would stick. Think of putting a sticker on regular copy paper “matte finish” versus sticking it to a “glossy” sheet.
I don’t know much about other glossy promos/Japanese cards that were made not as peel aways so I won’t comment on those. But maybe they just used the same stock?
– Well actually now that I think of it- early cards were released as “vending series” expansion sheets and they also were “peel away” sheets. This is how alot of people first got access to pokemon cards. bulbapedia.bulbagarden.net/wiki/Vending_Machine_cards_(TCG)
Unpeeled CoroCoro promo back/front:
Hope this was maybe a little helpful??
–caveman
EDIT: if you havent peeled one these promos yourself you can pick some up cheap on ebay. www.ebay.com/itm/POKEMON-JAPANESE-POCKET-MONSTERS-COROCORO-CLEFFA-PROMO-CARD-UNPEELED/352904672117?hash=item522ac1f375:g:iAAAAOSw0NZeANRs
Here’s a TCAgaming unpeeling a set of vending machine sheets on Youtube. You can hear the stick as he peels them off: Complete Vending Series 3 UNPEELED! Large Purchase - YouTube