The 1x3x3 door with a window was produced in two distinct versions. The earlier piece was molded in one piece from clear plastic that was then painted. It first appeared in sets in 1967. Later, LEGO switched to a two-piece construction, with a clear part and a colored part fused together. The orientation of the door handle was also changed from vertical to horizontal.
The painted pieces are especially susceptible to wear, as the paint can scratch off. There were also sometimes production quality issues such as bubbles in the paint that would flake away.
The top left door is a two-piece version with horizontal handle. The other two blue doors are painted versions, with the vertical handles. The painted versions are only painted on the front and part of the way around the post.
The yellow doors illustrate some of the condition issues common to this particular part. The yellow door on the left has a paint bubble that occurred during production and has flaked away. The door on the right shows some typical playwear that has scratched and worn away some of the paint.
The painted doors were released in yellow, red and blue. The two-piece colors are known in the same colors. It is difficult to tell when the change occurred, as Bricklink’s inventories are not reliable for this variant. It is probable that by 1973 the new versions were appearing in sets.
The 1x3x4 door was also produced using the painted decoration method, as shown by the ambulance doors below. The larger door was never released in a two-part version.