I haven't seen this before but I suspect it was worn by a British 'War Worker' - munitions factory worker, coal miner or other occupation deemed necessary. Worn during the war to explain why a seemingly healthy male was NOT in the army. There was huge pressure on English men to join up, including being handed a white feather [mark of cowardice] by young women on the streets. This insignia would protect one from that and other criticism. Probably worn after the war too, to mark one's contribution to the war effort and final victory.
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