This is all I could find:
AM5/15B
Channel Amplifier. Channel amplifier used as subunit in PA8/411. Provides 1 pre fader and 2 post fader outputs. It is mechanically and electrically compatible with the AM5/15 but has 9dB gain in pre-fader output. Powerrequirements -24V dc. PCB 101.5 x 89.0. Parent equipment PA8/411. SCPD 1981.
I can't find the PA8/411, but here is the 410:
Desk Panel for Channel Facilities for single mono channel.Provides microphone amplifier with 5dB stepped gaincontrol, RSA (AM1/4 incorporated into panel) PA, FB, Echooutputs, 2-group switching. Designed for Local Radio Mk.3Ainstallations. Module 39mm x 338mm x 230mm with Variconconnectors. SCPD 1977.
I think a microphone amplifier would tend to be an AM9.