Richard is correct they were used on convoy vehicles that were stopped for whatever reason to tell the rest of the convoy to keep moving and not stop.
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Cheers
Cliff Hutchings
aka MrRoo S.I.R.
"and on the 8th day he made trucks so that man, made on the 7th day, had shelter when woman threw him out for the night"
MrRoo says "TRUCKS ROOLE"
