Michael Ernst claims this sentence is ambiguous. Read one way, (41) means that the concatenation of the first letter of the Roman alphabet with the second letter is an expression; read another way it means that the expression between the two outer quote marks is an expression.  Confronted with this ambiguity, Boolos (1995) introduced a notation for quotation in which every quotation mark ‘knows its mate’.  So understood there are denumerably many distinct quotation marks, each formed by prefixing a natural number n of strokes to a small circle, and in order to meaningfully enclose an expression with Boolos quotation marks we have to choose quotation marks of a ‘higher order’ than the quotation marks that occur in the expression to be quoted (if there are any) and each quotation mark in a grammatical sentence is to be ‘paired’ with the next identical quotation mark (Boolos 1995, p. 291). Accordingly, reading (41) as containing Boolos quotation marks, we get the more natural reading according to which it says that the first letter of the Roman alphabet concatenated with the second is an expression.