Some on this thread have said the dealership stamped warranty blocks.
I've been to over 100 dealerships over the years from 1975 into the 90's... in their stockrooms, upstairs, downstairs... all over.
A couple members here have been to more dealerships than me I'm sure.
In my travels I've yet to stumble onto anything that resembles any stamps in dealership inventory.
There is no part number that exists for such, so where did they come from.
Was there a bulletin telling the dealerships told to go to Napa or somewhere and buy stamps if needed.
Perhaps they were kept in the vault.