Electronic Serial Number Upgrade Module for Pactor PTC-II and PTC-IIe modems manufactured prior to June 2003.
Pactor 3 upgrades for modems manufactured prior to this date were referenced to the call sign of the modem owner. This made it impossible for the original owner to sell the modem with a Pactor 3 license.
Modems manufactured after this date incorporated an electronic identification board that referenced the Pactor 3 license to this serial number, this made it possible for the owner to sell the modem with a Pactor 3 license.
If you are in doubt, check the modem serial number. Twenty-two (22) character serial numbers will have the ESB board installed.