5.4733 Frege says: Every legitimately constructed proposition must have a sense; and I say: Every possible proposition is legitimately constructed, and if it has no sense, then that can only be because we have given some of its parts no meaning.
(Even if we believe that we have done so.)
So “Socrates is identical” therefore says nothing because we have given no meaning to the word “identical” as an adjective. Since if it occurs as the sign of equality then it signifies [symbolisiert] in a wholly different way – the signifying [bezeichnende] relation is different – thus the symbol too in each case is wholly different; the two symbols have only the sign [das Zeichen] in common with one another, by accident.
Marie McGinn (p. 242) says that the reference here is to §32 of Frege’s The Basic Laws of Arithmetic. She adds (same page) that “Wittgenstein’s disagreement with Frege amounts to a reassertion of the context principle, and thereby of the priority of the concept of the sense of a proposition over that of what the constituents of a proposition signify.”
Black also cites vol. 2, §92 of Frege’s book.
My comment: Frege allows propositions (and thoughts?) that are not properly constructed. Wittgenstein says these either are properly constructed or else not propositions at all. Meaning depends not on proper construction of propositions, but on propositions having parts that have been given meaning. It is arbitrary all the way down: what counts as a proposition in our system and which words have been given definitions.