The letter, postmarked Houlton, Maine was recently delivered to Pittsfield when it mysteriously turned up after so many years.
The nine-page letter, written in ink in luxuriant cursive, was penned by then-23-year-old Miriam McMichael, a school teacher, to her mother, Dollena McMichael, who lived on Peltoma Avenue in Pittsfield.
Both women - Miriam and Dollena - have since died.
Postal officials in Pittsfield found the letter in the morning mail and contacted Skowhegan Postmaster, knowing that McMichaels had moved to Skowhegan some 40 years ago.
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Ann, 69, is Miriam's niece and Dollena's granddaughter.
The spelling of the family surname has changed over the years.
Ann said part of the irony of the mystery letter is that her aunt is apologising for taking so long to write to her mother, with whom she was very close.
Michelle Rowell, a clerk at the Pittsfield post office, said she found the letter in remarkably good shape while hand-sorting incoming mail.
"It had a two-cent stamp on it. I knew it was old and the handwriting was old," Rowell was quoted as saying by 'centralmaine.Com'.
She said it probably slipped 'neatly' into some old sorting equipment somewhere along the line.