via u/runslack at https://www.reddit.com/r/bulletjournal/comments/18rghxp/comment/kg8fv4t/

I see, that’s nice. I asked for a7 format since I want a portable « device ». I currently am following the today system method with my stuff from https://www.x17.de/de/mind-papers/karteikarten-lernkarten-karteikartenbox-karteikartenkasten-lernkartei-mindpapers/.

X17-Mind-Papers - die Wiederentdeckung der Karteikarte<br /> https://www.youtube.com/watch?v=wxZMia35usc

Mind Papers has a variety of small leather covers (folders) with binder clips for storing one's note cards. They range from smaller than A7 up to A5 sizes.

They're broadly reminiscent of smaller versions of the Everbook, though I suspect these came first given the 2014 post date.