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They were the bulbs in Chinese 125's. Seem to remember them being a bit crap but good enough to not ride in to a tree at night.

BA20D? That's the only bayonet headlight fitting I know. Usually used on cheap off-brand 125cc bikes.

3 Tangs sounds like H4.

Picture would be great but you describe a holder.... if the holder is separate to the housing then it maybe a H1 https://en.wikipedia.org/wiki/H1_lamp