In case it helps, the Social Security death index does not give a place of death for anyone. It may give a place of last residence or a place where they last got their benefits, which may or may not be where they died.
There is an online death index for Massachusetts that goes up to 2003. I don't see an entry for your Margaret in there.
Another transcription of her entry in the Social Security death index says her Social Security number was issued in Massachusetts, and her last residence has the code of 900, which refers to the U.S. Consulate, Dublin, Republic of Ireland. Is it possible she died in Ireland?
The claim file under her name says that she made a claim on 7 April 1969 in the Cambridge, Massachusetts Social Security field office.
https://aad.archives.gov/aad/record-detail.jsp?dt=3139&mtch=1&tf=F&q=025014842&bc=sd&rpp=10&pg=1&rid=1566154 Considering her age, that was probably a claim for retirement benefits.