What does "Answers on a postcard" mean?
It used to be common for magazines in Britain (and over the Atlantic for all I know) to print in each issue a conundrum or quiz question 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 send than a letter, and postcards were easier for the magazine staff to sort, so the standard instruction was "Answers on a postcard, please, to [the magazine address]". So this became a catchphrase for "If anybody knows, please tell me".
DJK