What was a wealthy upscale Boston neighborhood in late 1800's?

The Back Bay. Was then, is now. The South End was actually built up as housing for the servants of Beacon Hill and Back Bay families. According to W. Firey in his 1947 Book, "Land Use in Central Boston" (Harvard U Press), The Social Register listed 1,922 households of which 1,519 were in the city and 403 in the suburbs (mainly Cambridge, Newton, and Brookline).

Of the 1,519 households in the city of Boston, 867 were in Back Bay, 280 were in Beacon Hill, and 56 in Jamaica Plain. By 1988 There were 2,732 households listed in the Boston Social Register but a mere 111 found in Back Bay and 128 in Beacon Hill. These numbers were eclipsed by Cambridge (209 Households), Chestnut Hill (153), Manchester (158), Dedham (143), and Brookline (132). Wellesley was close with 102 households in 1988. These statistics are from my book (Stephen R. Higley Ph.D.), "Privilege, Power, and Place: The Geography of the American Upper Class" published in 1995 by Rowman & Littlefield.