The doors on a Trident were different from the rest of the type. so it could be this door which is being shown.
The center door was a detactable type, in an emergency it was opened & dropped out. It was a bit smaller than the other doors.
There was a famous time at LHR when the cabin crew were alerted to a possible emergence evactuation after landing. On touchdown the crew opened the door, it fell out & the aircraft ran over it. It was then that it was found that the doors were not interchangable & it had to be rebuilt!!
Sorry for thread drift!!
tristar 500