Yes, these are the library book pockets you remember. The manila color makes them easy to alter if you choose. Or, use them just as they are. There is no adhesive on the backs, so you'll have to glue them if you choose to use them for their original intended purpose - as a slot for a library card or date due slip!
These medium pockets measure 3-1/2 inches by 4-1/2 inches. They are made from the same type of paper as lightweight manila file folders.