The side door, which connects to the docking tube or opens to the alien surface. Image by Andrew Novinc. Compare to the War Games image below, but this is a composite of details, explained below.
The side doors have no airlock, so to EVA into space the entire module must be depressurised (Space Brain, The Exiles, Bringers of Wonder part 2). Fortunately boxes, lasers and other loose items do not float out the door when the astronaut leaves the module.
In early episodes, there are two grilles either side of the door, with a small yellow control panel beneath each.
In Collision Course an additional large black door control panel appears on one side, which is sometimes used to operate the doors. It has 9 buttons (top row red, middle green, bottom blue until Space Brain when they become yellow)
By War Games the computer wall has been added, with shelves on the right of the door. In Year 2 there will be a white ventilation panel on the base, matching the one on the left, but it is absent throughout Year 1.
In Space Brain, the control panel on the left now has yellow buttons on the bottom row.
From The Metamorph, the set is revamped. There is now a white ventilation panel at the base of the shelving on the computer wall. The metal grille and small yellow control panel are gone from both sides. On the left side of the door there are no details at all. On the right is a large yellow label (a grid of Letraset letters), a small round red label "6", and a small yellow label. The black control panel on the left is retained, but has blue buttons on the bottom row (again).
In The Exiles, a metal rail is placed above the door, to attach ropes for EVA.
In the elongated version for All That Glisters.
We don't often see the port-side door, but it is used in New Adam, New Eve. It has the control panel on the left side only, with no other labels. It has a matching metal rail for EVAs above the door, like the starboard door.
It looks the same in Seed Of Destruction.
In A Matter Of Balance the black door control panel changes to a standard Alpha door panel.
In Matter of Life And Death, the exterior doors gain slots either side for the hand rails (and the first use of steps). The exterior side doors in Earthbound have a "airlock control", which is still present at the end of the year 1. By year 2, the exterior is bare.
Behind the scenes from Matter of Life And Death showing the exterior of the passenger module.
Copyright Martin Willey. Thanks to Marcus Lindroos.