3.318 Like Frege and Russell I construe a proposition as a function of the expressions contained in it.

Perhaps there is a likeness, but surely there are also many differences. The problem with "a function of the expressions contained in it" is that it then becomes unclear what a proposition is: this seems a per obscurum explanation at best.

Besides, what Frege and Russell maintained is, much rather, that both the meaning and the truth-value of a proposition depend functionally on the terms of the proposition, more or less in the sense that if one knows the meanings of all terms and the truth-values of all propositions that do not contain logical operators, then one can infer the meaning and the truth-value of the proposition. (The general idea coming down to: One knows what "cow", "eats", "grass", "sun" and "shines" mean; so one knows what "the cow eats grass and the sun shines" means, and since it is night one knows "the sun shines" is false and therefore so is the conjunction even if "the cow eats grass" is true.)