Thanks Rob, Jochen, Pete for your help.
Baydel tech support emailed me back quite quickly to let me know that
the card is a "CTS-11", and was designed to replace a TS11 +
controller with a QIC drive (there was a disconnected QIC drive in
the Dyna5 chasis, but it was so corroded, dirty & yuk that it's in
the heading for the bin pile)
The chap at Baydel (Leatherhead, UK) said he'd been there for years
(25 or more) and had seen them through making whole PDP11 systems
(apart from DEC CPUs) to their current range of NAS & SCSI RAID etc
cheers
greg