Back to 1916 already... And not too surprising to see it in connection with railway safety, the railways have always been safety innovators.
Should we offer a prize for the first person to find a reference to one made at St Paul's Chuchyard ?
Phil
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I'm told that it was common in British coalmines in 18-19C (they were all privately owned and independent then) to use triangular padlocks for safety purposes. The distinctive shape could be easily felt in darkness, and understood by illiterate workers. The triangular padlcok body was not uncommon, though that does not definitely prove anything..