The author of the "Rasmussen" cable was someone who had access to Coast Guard communications, and could sign his last name in confidence that the sender would be known, so---"Rasmussen" was most likely Lieutenant M.W. Rasmussen, Commandant of the Fifth Coast Guard District, which covered New Jersey.
How Lt. Rasmussen obtained to this information is unrecorded. He may have been aboard or received a radio message from a cutter or a merchant ship, or from the master of a ship that docked in the Fifth District.
Decre writes that a coast guard patrol boat, CG-234, recovered one wing. The most plausible hypothesis is that somebody looked at the wing and decided it wasn't interesting. In any case, after three months, the wing could have come from most anywhere. L'Oiseau Blanc probably lacked the fuel to get much farther south than Boston, and inshore currents south of Newfoundland would tend to carry debris into the Gulf of St Lawrence.
adr