NEHGR 12:345 is about Robert Pond's will. Mary, his widow. No surname.
NEHGR 19:36 is about the will of Nicholas Ellin, which mentions his wife Mary, and a couple of her Pond daughters-- i.e., so Mary, wife of Pond,m2 Nicholas Ellin. Nothing about her maiden name.
History of Milton, p 23, is about Nicholas Ellen/Ellin; mentions that his wife was widow of Robert Pond.
Blake-Torrey genealogy, pp 42-43 has a short bio of Robert Pond; nothing about her origins or maiden name; says she subsequently married Edward Shepard.
The Pond genealogy, if accurate (published 1873) would seem to suggest that we have conflated two generations--that there was a Robert Pond, immigrant, who died in 1637 [error; subsequently proven to be about 1647] (wife Mary m2 Edward Shepard), and that Robert and Mary had a SON named Robert Pond of Milton (made freeman 1642) who married Mary Ball (and that it was she who m2 Nicholas Ellin). This son Robert died btw 1660 and 1663.
The Clapp genealogy is about Robert and Mary's daughter who married a Clapp. There is nothing about Mary's origins.
I have checked the other sources from Torrey for the first listed Robert Pond and there is nothing about her origins.