Answers on a postcard, please

At one time (long ago, in my youth) newspapers and magazines in Britain used to publish conundrums or quiz questions for their readers, with a small prize for the first correct reply, or the first 10 correct replies, or whatever.

Until a few years ago a postcard cost less to mail than a letter, and postcards were also easier for the magazine staff to sort through (no opening of envelopes), so the standard instruction was "Answers on a postcard, please, to [the magazine address]". So this became an idiom for "If anybody knows, please tell me".