OK, times up.
The USMC AV
-8B originally being of Brit origin, the integrated boarding ladder is on the righthand side. Why? I once asked a Hawker Engineer that question. He told me its a tie to British Calvery mounting their horses from the righthand side. The Harrier is the only USMC
jet aircraft of British origin. It is also the only one that is boarded from the right.
All helicopters with side by side seating have the pilot on the right. This dates back down to early helos which had only one collective stick between the two seats. Since most people are righthanded and prefer to have their dominant hand on the cyclic, the right seat became the pilot seat.
With tandem seating helos, climbing into a crew station over the collective lever is a pain. Therefore ingress on the right is prefered. The forward gunner seat has ingress on the left for canopy structual reasons. If the collective lever is up while ingress/egressing, the gunner just has to take necassary precautions with private parts.