I was at Nakhon Phanom, Thailand in 1967. There was a Navy unit there with P2Vs, they were used to make precision low level drops of sensors near the Ho Chi Minh trail.
They had a chopped off tail and small auxilliary jets out near the wing tips.
Of course at the time we weren't given a clue about what they did. We just thought a Navy aircraft at a air force base that far from the sea was strange. One went down over Laos, it's whole crew of 12 still missing.
I never found out about Igloo White and Nakhon Phanoms part in it till years later.