What does "The black spot is under the plate" mean?

Simon Jenkins, in The Guardian online newspaper, writes:

How can the British Tommy, the jolly tar or the air ace confront the dreaded foreigner, when a defence secretary cannot say boo to George Osborne? Be a man, Fox, they cry. The black spot is under the plate. The pistol is in the gents.

I googled for the exact phrase, and the only hit is this article. Googling for "black spot under plate" just gets me a lot of stuff about toenails. What on earth could he mean?

Solution 1:

Perhaps the black spot is an allusion to Treasure Island? Perhaps the meaning is that (Liam) Fox (defence secretary) should quit (i.e. accept the black spot).

Solution 2:

In 'Treasure Island', the black spot is a threat of imminent death delivered by a fellow pirate (invented by Stevenson, as far as I know). 'The pistol is in the gents' refers to the (probably equally mythical) practice of leaving an officer who had misbehaved alone with a revolver, and expecting him to shoot himself. So; Quit before you're fired, Fox!