Very interesting! Thanks for sharing this. I received the land case file from NARA but it contained no clue about Purley's family. Oh, well -- I knew it was a long shot.
There is a very interesting book about the history of Watertown, MA that might interest you. It's called Divided We Stand by Roger Thompson (University of Massachusetts Press). The Jennisons are mentioned several times in the book and while the focus is not genealogy it's an interesting look at what life was like in Watertown from 1630 to 1680.