Margaret W. Brooks Letter Envelope (Longtime Secretary of Morse)

 The envelope is postmarked April 21, 1927, and it is addressed to Margaret M. Brooks. However, there seems to be an error as the sender mistakenly typed "M" instead of "W" as her middle initial. Margaret, born in 1860, was a daughter of Henry M. Brooks (1822-1898), who authored the Olden Time Series https://www.gutenberg.org/files/17483/17483-h/17483-h.htm. It's worth noting that Edward Morse wrote the introduction for Brooks' Olden-Time Music, published in 1888. Thus, the relationship between Morse and the Brooks family is unquestionable. Additionally, there is a bookplate within the Peabody Essex Museum collection that Edward Morse designed for Henry Brooks (see attached photo).

Margaret is listed in the 1915 Salem Directory as Margaret W. Brooks, residing at 60 Ocean Ave., Salem, Mass. She shares this residence with her sister Alice. Notably, Alice was the manager of the School of the Museum of Fine Arts, Boston. for many years. Alice also held a title at the Essex Institute. Another resident is Jenny, associated with the Jenny Brooks Company, which specialized in selling Original Designs for Embroidery and Cross-Stitched Patterns.

The artwork on the reverse side of the envelope poses a mystery, as does the reason why this particular item was discovered within the Morse estate. Both of these mysteries remain unsolved.

UPDATE:

Oddly, Peabody-Essex-Museum shows her with the initial "M": Scrapbooks was compiled and organized by Margarette M. Brooks, Morse's secretarial assistant from 1878 until 1925.  https://pem.as.atlas-sys.com/repositories/2/resources/182

However, the digital read on https://www.aap.amdigital.co.uk/introduction/nature-and-scope shows a "W": The Scrapbooks series was compiled and organized by Margarette W. Brooks, Morse’s secretarial assistant from 1878 to 1925.

Conclusion: 

From the letter published on https://content.wisconsinhistory.org/digital/collection/p15932coll7/id/5895/rec/1 it clearly shows Margarette uses "W" as her middle initial (see photo). 

Margarette Ward Brooks (1860–1929) is mostly known today as a secretary to Morse, but she was also a scientist in her own right. She wrote "Influence of Temperature on the Chirp of the Cricket" in Popular Science Monthly in 1881 when she was just 21 years old!