Dummett's Backward Road to Frege and to Intuitionism - Tripod

As Frege explains in a third text in another letter to Russell:
Now a class cannot be the sense of a sign, but only its meaning [i.e. reference], as Sirius
can only be the meaning of a sign, but not its sense. Hence a class cannot be the indirect
meaning of a sign, any more than Sirius can....Can any class whatever be a component
part of a thought? No more than the planet Jupiter can. A class...can be defined in
different ways....If the class was part of the thought that an object p belonged to it, then
the change in the sense of the class name would not affect the thought, provided that the
class itself remained unchanged....
Or does a proposition have a meaning [i.e. reference], and this is a thought? If
the latter, then the propositions ‘2 3 > 7’ and ‘3 2 - 1 > 7’ would have to designate the
same thought; [for they have the same reference]. Now the thoughts contained in those
propositions are evidently different....We are thus compelled to regard a thought as the
sense of a proposition. (1980a: 157, my emphasis; see 158)
Note that logical equivalence is not the test of identity of thoughts. The natural reading is that since
objects are customary references, and customary references remain the same across changes in sense,
no objects are senses. That is, the most general reason why objects are not senses is that all customary
references, including objects, are extensional in sense (2) and senses are intensional in sense (2). Frege
is telling us that we do not understand his solution to his puzzle about informative identity statements
unless we understand that no senses are objects or functions.
Frege repeats this in a fourth text in “[Notes for Ludwig Darmstaedter]”:
[T]he object and concept are not constituents of the thought expressed....A distinction
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