A 300/1200 baud modem sold with, or designed to be used with the RS232 unit (below), both made by PACE using Commstar software.
Below pictures and video show the unit connected online to Arcade Demon in 2013, the last BBS operating in the U.K. (as far as aware) compatible with an Amstrad CPC. The site was aimed at Acorn 32 bit users, and stopped updating around 1995.
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Video (You Tube): [https://www.youtube.com/watch?v=3WPMq80wYFY Guide to Getting Online with an Amstrad CPC 464 (from 1984) in 2013]
Online with TTY screen display at 300 RX=TX Baud with 0 Parity, 8 Data Bits, 1 Stop Bit, Soft Handshake.
=PACE RS232 Serial Interface with Extra ROM Board=