it looks like all the lettered stands have been more recently grafted on to an existing terminal. Is that correct?
Most definitely. Initially I'm sure pax were walked across the ramp to their jets. Then the airline built a thin, c-shaped structure to shelter them.
JFK has seen every era in modern aviation, from piston airliners to the 380. At some point maybe they should have lined up bulldozers side-by-side, scraped the whole thing off into the bay and started over. But they did something else - just made it work as they could.
I remember the TWA terminal with its stunning jetways that could reach over the wings of a 747. Now it is surrounded by A320s....