H.F. Walling, 1858 (BPL)
In 1851, the outlying district of Roxbury seceded and formed the town of West Roxbury. This map segment shows the north border between the two towns. From the bottom right, the border starts at Seaver street and today's Blue Hill avenue. That means that all of Franklin Park was within the town of West Roxbury. From there, the border follows Seaver street west to Washington street. Notice that there's no Columbus avenue at Egleston square yet. From there, the border continues in a straight line, just north of Boylston train station, and through Hyde square, continuing in a line to the Brookline border at Muddy river.
So there you have it. The one time you could say Jamaica Plain had an official border, none of Centre street east of Hyde square was part of it. Heath street, Day street, Bynner, Wyman, Mozart, Bickford; all paid taxes to the town of Roxbury. And the same streets became part of Boston when Roxbury was annexed in 1868. My impression is that only when West Roxbury - with Jamaica Plain - voted for annexation in the next decade did the area just south of Parker Hill would start to consider itself part of Jamaica Plain.
It would be nice to know how they decided to draw the border line when West Roxbury seceded from Roxbury. Did the people north of Boylston street ask to remain with Roxbury? Did the Jamaica Pond gentry want to get rid of the Heath street crowd? And how did they work out taxes when the border cut through properties in a straight line? Every time you learn something, it just brings up more questions.