Probably the majority of SFP-marked Zuckerman helmets I come across have the letters added individually. This example has a water slide transfer decal added.
N.B. "S.F.P." is usually ascribed to the Street Fire Parties (introduced as part of the Fire Guard Organisation in August 1941) but there were Supplementary Fire Parties as well (that were created in 1937 and ran through until August 1941). See a Short history of Fire Watchers and the Fire Guard Organisation during WW2 for more information.
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