Katherine Hepburn had a very similar accent; however, yes, it is a Long Island one.
Re your query...was that aimed at me?
Nope, didn't live in Locust Valley ever, but my family has our summer house nearby. and NO, I don't sound like that! :-)
But yes, you nailed it...Thurstan Howell had a LOCUST VALLEY LOCKJAW accent on GILLIGAN'S ISLAND.
So is her accent.