Apart from the threads already mentioned by @thymeee , these two threads are useful as well:
www.elitefourum.com/t/mewtwo-wb-gold-stamp-movie-promo-japanese-ver/22725/1 Just like the the English Shadowless Base Set yellow cheeks Pikachu has two different prints, one in the US Shadowless Base Set and one in the Japanese Pokémon Song Best Collection CD (the one printed in Japan has thinner cardstock), a similar difference can be found for the English WB Mewtwo promo, which has a Japanese release that was included in the VHS.
www.elitefourum.com/t/super-energy-retrieval-non-glossy-japanese-promo/19503/1 Super Energy Retrieval has four versions: two glossy (Fan Club promo and Pokémon Best Song Collection CD promo); one non-glossy (unknown origin); and one from the Neo Genesis set.
Later in that same thread KEI also mentions how to distinguish the four glossy cards (Mewtwo, Mew, Super Energy Retrieval, and Computer Error) from the Pokémon Song Best Collection CD promos with their earlier releases by looking at the cross-section of the card with a loop.
Greetz,
Quuador