Capital "A"and"Capital A"mean two different things in the context of this old architecture diagram.
The first refers to the Roman letter "A."
The second refers to a Greco-Roman article (the column's upper plate, or "capital," labeled "A").
This is an elegant reminder of the power we have in punctuation. It's not just the quotation mark that works this way. An apostrophe is the difference between hell and he'll, between can't and cant, a fancy word for cliche or jargon. And as Lynne Truss pointed out, extra marital sex is a good thing to have, while extra-marital sex is not.
At any rate, happy punctuating. (And thanks, Craig!)