One of the later versions of the opening lines became:

When Mr Bilbo Baggins of Bag-end, Under-hill, announced that he would shortly be celebrating his eleventy-first birthday with a party of special magnificence, there was much talk and excitement in Hobbiton.

THE SECOND PHASE. XIV. RETURN TO HOBBITON.

So it's very possible that the first occurence of "When M..." was meant to be "When Mr Bilbo" etc.

One of the later versions of the opening lines became:

When Mr Bilbo Baggins of Bag-end, Under-hill, announced that he would shortly be celebrating his eleventy-first birthday with a party of special magnificence, there was much talk and excitement in Hobbiton.

• The History of Middle-earth, Volume VI, The Return of the Shadow, The Second Phase, Chapter XIV, Return to Hobbiton

So it's very possible that the first occurence of "When M..." was meant to be "When Mr Bilbo" etc.