Hey, I opened tons of the Amada Gold packs and gave some away along with many other series graded quite a few incl. the unstamped Mew one your asking about year or two ago. Im not certain if they were randomly inserted or if there was a special way of obtaining the gold letter stamp.
There were a-few people on Whatnot who were experts on these who sold breaks on sealed Poker Decks similar genre Pokemon cards (niche) obscure. Google lens can give you some additional info and Meiji also made similar ones there all hard to get high grades because mirror finish is so easy to scratch even from pack.
Hiddentemple on ebay is one of the experts there’s some others from my understanding. I personally focus on Pokemon comic books/mags/appendixes/mangas myself so no longer do the stickers. This is where people post stickers Amada / Meiji / Poker etc. I have way back as well trying to get more info on a few sealdass folding cards.