Mueller wrote this letter to Barr on March 27.
Barr has some ‘splainin’ to do. This letter indicates that AG Barr apparently lied to Congress in April when he claimed he was not aware of Mueller’s position on his summary. Needless to say, Barr thought he was safe with this lie. He did not expect Mueller’s letter to become public, since nobody in Mueller’s group ever leaks anything.
But there’s an interesting sub-plot here. The WaPo’s release seems have been created from Barr’s copy of the letter, not from the original. (There is a hand-written acknowledgement stamp, indicating that it was received on March 28 by the OAG.). Barr seems to have a snitch in his own department.