Pre-release Promos
These cards were "pre-released" and sent to store owners before the sets came out.
They served as "advertisements" to convince the store owners to buy the sets to sell to the public.

Most cards look similar to the normal cards in the set, but the difference is on the back.
It either says "prototype" or NOT FOR RESALE"
It might also include a description of the set that the card is promoting.