League Promos and League Challenge Promos!
League Challenge Delibird and Pokemon League Snorlax for examples;
These were set cards with a unique Cross Hatch reverse holo pattern given out at Play! Pokemon’s Pokemon League events starting in 2009 and League Challenge event starting in 2013 respectively. League Challenge promos on top of the reverse holo pattern got a stamp signifying your placement in the tournament 1st-4th as you had to win them.
Pokemon League promos were simply given out to participants. They became less interesting in the future only having the league promo stamp and the basic reverse holo effect, but League Challenge promos kept there 1st-4th placement, though losing the cross hatch pattern in 2015 with the release of BREAKthrough. They unfortunately have not been seen since Ultra Prism in the SM era.
Pokemon League promos stopped at early SWSH, but were brought back in 2022 with Play! Pokemon Prize packs. These cards have the stamp and some are holo including cosmos and a silver border in SV era packs. With the SWSH holo effect on certain cards in SWSH era packs. These promos also lost the cross hatch pattern with the release of BREAKthrough. It is interesting to note however, that for the BREAKthrough set promos they did not go with the normal reverse foil at the time instead opting for a mirror foil like patternthey have done in the past before going to the XY reverse holo pattern come BREAKpoint promos. This is present on both the Pokemon League and Pokemon Challenge Promos
Flabebe BREAKthrough Promo
Marowak BREAKthrough Promo
Chikorita BREAKpoint Promo
Greninja BREAKpoint Promo
Prize pack info
https://www.pokemon.com/us/pokemon-news/support-your-local-game-store-and-receive-play-pokemon-prize-packs
Full List of League Promos(Not including prize pack cards there is a great thread about that here)
Full List of League Challenge Promos
Wanted to challenge myself with the research here so hope you enjoy