Seems like the family started losing control of M&H in the "early" 20th century. The American Piano Company, which purchased M&H in 1924 was based in Rochester, but M&H pianos continued to be built in Boston until the Aeolian takeover.
From Wikipedia......"The Cable Company, a Chicago piano manufacturing company, owned an interest in Mason & Hamlin from 1904 to 1924, when it was sold to the American Piano Company. Mason & Hamlin's role in this company was later described as the "artist's"' brand among the firm's premier lines which included Chickering and Sons ("family use") and Wm. Knabe & Co. ("Metropolitan Opera's favorite"). American's sales began to decline in 1928, and following its collapse in the wake of the stock market collapse in late 1929, Mason & Hamlin's trademark, inventory and equipment were sold to American's competitor Aeolian for $450,000 while the factory buildings were sold off separately by the end of the following year. In 1932 it became part of Aeolian-American when the two companies merged, which consolidated the control of more than twenty brands of pianos; Mason & Hamlin, which had been at the former Hallet, Davis & Company piano factory in Neponset, Massachusetts, was moved to a separate plant at the Aeolian-American complex in East Rochester, New York at this time."
Even during the Aeolian era, the plates on M&H grands (as far as I can tell) say "Mason and Hamlin - Boston." That is still the case today - even though the Haverhill plant in 40 miles from Boston.