Not sure what you mean by a letter stamped on each one. Do you have a picture. Are these used inserts that came off a game or were they made for game and never used? I would think new insert would be blank.
I usually heat up the old inserts pretty good with a hair dryer and use a nut driver and tack hammer to tap them out. To put them in I use Gorilla wood glue. Moisten the playfield wood, where the insert will go, with some water and then put some Gorilla glue on the wood and the edge of the insert. Gorilla glue is activated with water for bonding. Once installed, I used a plastic block to level with the playfield. Carefully wipe away any excess glue. After 10 minutes check the insert for level and adjust if necessary.