Here are the some of the private stamp printing firms
who have produced stamps for the USPS.
American Bank Note Company (ABNC) PB inscription = A
+ plate numbers
Sennett Security Products (SSP) PB = S
Avery Dennison (AVR) PB = V
Banknote Corporation of America,Inc. (BCA) PB = B
Ashton Potter (USA) Ltd. (APU) PB = P
The last two BCA and APU presently print all US stamps.
You can find this info going through all the Postal Bulletins
from 2001 to 2020 https://about.usps.com/resources/po...bulletin.htm
Prior to those years you can check here:http://www.uspostalbulletins.com/pd...oup=48&id=48