This is a super minimal holder for tap cards such as arcade cards (taiko, DDR, Mariokart) and credit/reward cards (Timezone and other arcades), where you generally have the cards on a lanyard but need to scan them individually.
I couldn't find a card holder that wasn't super bulky or over-complicated with moving parts, and that let the cards stack as minimally as possible while also being easy to scan them individually without taking them out of a card-wallet or fiddling around.
A keyring or lanyard clip through the holes then stops the cards from falling out.
We use these very regularly at our local arcade and it has worked great.
Put some tiny dots of glue along the channel on part A, and then put part B on and put something heavy but extremely flat on top of them to clamp them while the glue sets.
The author marked this model as their own original creation.