That's what Sir Charles Portland asked when looking at the slides at the outset of 'Do Not Forake Me Oh My Darling.' In fact number six, the sixth slide, turned out to be a badly over exposed slide of Professor Jacob Seltzman as pictured here.
And yet this appears to be forgotten by the time the Prisoner is sorting through the photographic slides in his house in order to break the code. Because this slide of Seltzman is placed above the letter "N" which is number 14 in the alphabet, and the last letter of Seltzman's name.
And of couse the slide itself is numbered 14, so to correspond with the letter of the alphabet, and therefore Seltzman's name, and the code. So what's Number Six?
Be seeing you