The "hidden pocket" found in the 3rd and 4th generation Tenuis and Tynd models. It's more accurate to say that they modified an existing pocket layer so that it could be used as a hidden pocket, rather than adding a thick pocket layer specifically for this purpose.
Many of Sorahanpu's wallets have a structure where the outer leather/canvas is joined to the inner banknote compartment made of goatskin, which naturally creates a space between them. By making a cut in the goatskin side of the banknote compartment and adding a thin piece of cloth or auxiliary plastic parts, it can function as a hidden pocket.
Initially, it only held keys, then one slot held cards and the other keys, and now I'm constantly updating it so that both slots can hold cards and keys. Whether it's a thin wallet or not, don't you find it quite convenient to have a place to put cards and keys that you don't use often? Once you experience it, you'll understand how convenient it is to avoid losing your keys and to be able to go out with just this.
Tynd Small Slim Wallet
https://solahanpu.com/products/tynd-cv
Tenuis 3rd Generation:
https://solahanpu.com/products/tenuis-third
Tenuis 4th Generation:
https://solahanpu.com/products/tenuis-fourth