Specifically an R209 Mk.1 in Carrier, Set No.48.
See:
http://www.royalsignals.org.uk/R209-all.htm
It's the original late-WW2 6-volt version (later there were 12V (Mk.2) and selectable (via different power cables) 12/24V DC or 110/240V AC (Mk.2/B) versions.
Chris.