Statistics for occurrence #1 of “Middlesex county” in chapter 2, page 15 of Historic leaves, volume 2, April, 1903 - January, 1904:

...cality, is now extinct, and all that remains to mark the record of their lives are a few old gravestones in the ancient cemetery at Charlestown, and various wills and deeds in the Registry offices of Middlesex county . Much of story and romance is hidden between the lines of these old records, and in imagination one can call up vivid pictures of life in the old colonial days while poring over these old papers.
Max. Freq. Min. Freq.
Entity Corpus Doc Corpus Doc  
Middlesex County (Massachusetts, United States) 213 6 1 0 0 user votes
Middlesex County (Virginia, United States) 131 0 3 0 0 user votes
Middlesex County (Connecticut, United States) 0 0 0 0 0 user votes
Middlesex County (New Jersey, United States) 0 0 0 0 0 user votes

† This entity has been selected by the automated classifier as the most likely match in this context. It may or may not be the correct match.