Cryptic crosswords
I don't see any threads that mention this.
Part of the fun of this film is that, like the characters who would have figured out who had which fake-secret card if they'd looked at the picture, we have the answer to who the killer is, all along.
The title of the film, for anyone who does cryptic crosswords (like the ones in the London Times), tells us clearly.
These crossword puzzles have clues that look like one thought, but actually have two parts -- one half is the definition, as in any crossword, and the other half is often wordplay.
Here is an article by, of all people , Stephen Sondheim, about them:
http://nymag.com/arts/all/features/46798/
The clues almost seem like gibberish at first. But, after a while, they get almost predictable to people who do these puzzles frequently.
Any cryptic crossword lover would instantly know that the phrase "The Last of Sheila" meant the letter A.
The killer is standing under the last letter of Sheila in the picture.
So Perkins and Sondheim put in a little wink to their fellow puzzle-lovers.