Watching a WW2 documentary I noticed a man with an odd looking holster which hangs down at his knee! In this screen grab he is the one standing to the left of Montgomery: https://imgur.com/a/KcXrsNj. What gives?
In WW2, many European armies wore uniforms with belts that were much higher than where we wear belts today. As a result of this, a belt holster on the right was difficult to draw from since it was up so high, and these armies would typically use cross-draw holsters on the left instead.
In a tank or other armored vehicle, the commander is already squeezed into a narrow hole, and a straight draw on the right is easier than a cross draw on the left while in such a confined position. But then you’ve got the problem of the belt still being up too high. Hence, the dangly style holster. It’s just a practical way of making the pistol easier to draw.
Soldiers who were issued tank holsters would occasionally re-arrange them so that they ended up being cross-draw shoulder holsters with the pistol hanging just under the left armpit.