EXCLUSIVE

Donald Trump's camp claims the center armrest separating him from a fellow passenger is the smoking gun proving she's a liar ... but we did some digging and she's on solid ground.

Jessica Leeds claims she was sitting next to Trump on a flight from Dallas to NYC in 1979, when he got all handsy with her. She says all of a sudden the center console disappeared, and that's when the under-the-skirt groping began.

Trump's camp claims her story doesn't hold water, because the center armrest in first class was fixed -- not removable.

Leeds claims she was on a Braniff Airways Boeing 707, but we contacted Ben Cass, the founder of the Braniff Airways Foundation, and he says 707s were not in service from Dallas to NYC in '79.

Cass says the plane was actually a Boeing 727, and the armrest was indeed removable by simply pulling it vertically.

The armrest was held in place by pegs and Cass said it would take only 2 seconds to lift it off.

We also got hold of the Braniff International Flight Attendant Manual, which makes it clear ... the armrests, just like Leeds said, could disappear.