From early 1940 ARP and Civil Defence vehicles could use the left headlamp as an identification marker (the right lamp had a blackout filter added and the left was initially to be entirely obscured). Lettered masks could be fitted over the headlamp to allow the Civil Defence Services and Police to identify vehicles during the blackout and at air raid incidents. The lettering was in white except for the FIRE which was in amber.
The below image is from Trico that manufactured the illuminated letters.
The first set of lettered signs included:
ARP - Directing Staff
W - Air Raid Wardens
FAP - First Aid Parties & Mobile First Aid Units
A - Ambulance (stretcher cases)
A CAR - Ambulance (sitting cases)
R - Rescue Parties
DC - Decontamination Squads
RP/R - Repair Party - Roads
RP/W - Repair Party - Water
RP/G - Repair Party - Gas
RP/E - Repair Party - Electricity
M - Messengers
SP - Stretcher Parties in the London CD Region
FIRE - Fire Service
P - Police
Additional letters were added such as BTS for the Blood Transfusion Service, GPO for the General Post Office fixing telephone lines and GIS and GCU for Gas Identification Services and Gas Cleansing Units, respectively.
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