Oopd, forgot the link to the 1944 article: http://www.time.com/time/magazine/article/0,9171,932363,00.html
William S. Bennet represented the bulk of the Upper West Side, Harlem to Washington Heights (from 1905-1911), exclusively in Manhattan (then the 17th) and he and 2 other Manhattan GOP seats were captured by the Democrats in the anti-Taft 1910 midterms. The districts were reconfigured for 1912. Bennet ran again in a 1915 special in the 23rd (which was about half of his old seat, including Washington Heights and extended into central Bronx, excluding the South Bronx), but lost the following year (even with Charles Evans Hughes at the top of the ticket). It was redistricted again in 1918 (where Washington Heights and part of Harlem became exclusively the 21st and the 23rd exclusively the west and central parts of the Bronx).
You were correct in that there was apparently no redistricting from about 1918 until 1944. The lines Torrens won under in the February 1944 special over Bennet* were the same. The new lines for the 21st in November 1944 was extended southward along the Hudson (Upper West Side) and Harlem was wholly excised for Adam Clayton Powell, Jr.’s new 22nd. Those same lines were in place for 1946. In 1952, the last of Javits’ terms, the 21st and the 16th (formerly the 22nd) were streamlined a bit, but the 21st acquired more of Harlem.
You are also correct virtually all of Javits’ old district lay within Rangel’s today, but the evolution of the district followed to Nadler’s (the last of the upper part of Manhattan was apparently removed in 2002).
*As an aside, had Bennet won in ‘44 and been reelected for the same period as Javits, he would’ve served alongside his son, Augustus, who won a single term in November ‘44 in the 29th district (which was Rockland, Orange, Sullivan & Delaware Counties). Augustus was an odd one. He ran against Hamilton Fish (the FDR antagonist one) in the GOP primary and lost, but ran on the Democrat, Liberal & ALP lines and beat Fish in the general by a 53-47% margin, but officially aligned with the GOP at his swearing-in. He was beaten by Katharine St. George in the 1946 GOP primary.
That was an interesting article you linked, which showed Northern Blacks hadn’t been fully corralled onto the Democrat plantation and could still prove a pivotal swing bloc. If they were that politically sophisticated now, they’d never have ended up with the situation they’re now in, allowing themselves to be used and abused.