This is one of a big bunch of “holders” (pockets/wallets/folders) designed to fit the 2-1/2" diameter Art Coins or Coasters announced with the Coins, Coasters & Holders Kit.
This particular one is scored chipboard to make a FAT folder (with a deep pocket that can hold up to 3 coins/coasters). When folded closed it measures approx. 3" x 3".
Note that the covers overlap (they don't meet in the middle when folded closed). You could add a "button and string" closure to tie the folder closed (just an idea).
There are lots of extra photos above, including one showing one of these folders with the larger 4.5 Inch Dimensional Coaster Folder.190516NB GND 190428new 1904283inchnotkit 190509SK