Shankland of Case/Cleveland interviewed Einstein and asked this question.
As I recall, AE said he probably had heard of the MM experiment, but
couldn't be sure, although he had earlier said (at a meeting in Japan) he
had not heard of it. Shankland subsequently wrote a feature article in
either Physics Today or Scientific American on AE, discussing also this
issue. Sorry to be so vague, but perhaps this will jog another's memory
of the Shankland (now deceased) article.