While researching a new pick-up, I came across this thread.
the Northern route was short-lived because of the beginning of WWII in Europe in late 1939. Because the US remained "neutral" direct service between the US and London via PAA on the Northern route was terminated, and all TransAtlantic Clipper service henceforth went on the Southern route.
So apparently this leg from NY to Southhampton was quite short-lived. No backstamp, unfortunately.