The Apple modem that ships with the PowerBook Duo 210 and 230 is the Duo Express Modem. It isn't compatible with the 100 or 500 series PowerBook computers due to physical differences between the computers.
PowerBook 160, 165, 165c, 180, and 180c
The Apple modem that ships with these PowerBook models is the Express Modem. This modem isn't compatible with the PowerBook 100, 140, 145, 145B, 150 or 170.
PowerBook 140, 145, 145B, and 170
The Apple modem that ships with these PowerBook models is the PowerBook Fax/Data Modem. This modem is also compatible with the PowerBook 150, 160, 165, 165c, 180, and 180c.