Alfred Jules Ayer, in a conference where he exposes the achievements of Moore, starts it by saying:
"It is true that Moore diminishes the force of his position by adding that no one knows the correct analysis of these propositions of which we all know the truth, and still more by admitting as possible analyses interpretations of the propositions in question that one would suppose him to have ruled out of court, but I shall ignore this complication in the present context" --Ayer, Alfred Jules. ‘A Defence of Empiricism’. Royal Institute of Philosophy Supplement; London 30 (1991): 1. http://dx.doi.org/10.1017/S1358246100007645.
In what text, or conference, or where, does George Edward Moore confess that we do not really know those sentences that he so much defended that we knew?
I guess that in his mind he always thought about it, I am quite sure that he could not believe that his reply to skepticism was truly valid, but I have not found a place where he accept it.