Useful post I found...
Series 2/2a/3/90/110/Defender doors are all the same shape with the same
boltholes for the important bits. A summary of the door types is as
follows:-
Series 2/2a - 2 piece (except rear & side rear of five door 109). Only
significant change in late production was to the window lock. Door check
pivoted on door, slides on bulkhead.
Series 3 - same as S2 except door check is pivoted on bulkhead and slides in
door. Optional (standard in late production) anti-burst door locks,
interchangeable except they need a hole in the door skin for the keyhole.
Series 2&3 optional groove along bottom for door trim.
Very early 110. Same as S3 door except that the door check is pivoted on the
door and slides on the bulkhead pillar. Glass is arranged differently with
the join between the two panes sloped rather than vertical. This door is
still fitted to most military 90/110. Tops are interchangeable from this
door right back to S2.
110/90 to 1987 Fabricated frame one piece door with wind up windows. Much
thicker, very prone to rattles. New remote operation door lock and
different striker plate.
110/90 to date. Pressed frame one piece door, even thicker, push button door
handles, new locks and striker plate, common to other
Leyland/Rover/Landrover vehicles. Generally much better engineered, but you
lose a lot of space.
All the doors are interchangeable with appropriate locks and striker plates,
except you may have to cobble up the door check arrangement. Fitting the
late 90/110 doors to earlier models will cause problems with elbow room
unless you also fit a smaller steering wheel and move the outboard seats
inward an inch or so.