There very well may have been some confusion as to
whether or not they
could legally disclose the technical details of the modem. Which chipset
did the modem use?
The 2400 bps  part is the Rockewell chipset, along with amn AM7910 for
the slwoer speeds (things liek V21, V23). I forget what the
IIRC there's an 8250 on the board that connects to the Amstrac bus. The
serial side of that connects to the modem stuff., In other words it's an
exernal modem and a serial port on one PCB (minus the RS232 level
shifters). Actually, a lot of internal modems were like that,
 Not strictly 2400 baud, at least not on the telephone line side.