To be clear, I was #3 and I purchased coach but was allowed to pick a seat in the J part of of the plane when I purchased the ticket. I've had that happen before on this flight or a similar flight. (It is a LAX-BOS A321 and they send it BOS/JFK to kill some time while waiting for the next BOS-LAX flight.)
This plane does not (yet) fly LAX-BOS. AA sends it on JFK-BOS turns to increase its utilization between
JFK-LAX and JFK-SFO turns.