Arlington,
My experience in trying to research 19th Century records is that they are often unreliable, replete with errors in transcription, phonetic spellings, guesses made by the transcriber/recorder, and entries simply made for convenience; particularly for the latter when there seems to be no harm or consequence.
The previous owner to the houses on Louisburg Square that became part of the convent was a Mrs. Joseph Barrows, a widow from rural Maine who arrived in Boston with little financial resources, four young children, and what was clearly a great sense of business acumen
Mrs Barrows first owned and operated a boarding house across the street from the state house. One might reasonably expect that state legislators were among her boarders. She soon moved her boarding house business to Louisburg Square, and subsequently into a series of newly constructed buildings in the Back Bay. By the mid 1880s, she also owned and operated a mansion-like summer hotel and surrounding cottages on the shore.
Its a fair presumption that Mrs. Barrows, in the early days of her business enterprise, was probably financed by powerful politicians. Said politicians may have helped finance her move to Louisburg Square. To avoid questions about potential tax liability, said politicians may have instructed the assessor to record ownership of the square as the city of Boston. A harmless change from the city's standpoint, if taxes were not actually owed on the square, or the surrounding road.
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In any event, Mr and Mrs Heinz and the other property owners on Louisburg Square can afford property lawyers whose hours and billing rates are such that if there were defects in the title with respect to ownership of the square, such would have been identified, discussed, and chortled over long ago.