I found a blurry image on the web, I couldn't tell if it was an illustration or a picture of a kit but it showed them as leather brown squares, I am still of the opinion though they are windows, obviously not standard as theres no other evidence anywhere. As for their use, either rough bomb aiming or possibly to see the ground when landing. A pity theres no other information, they could well be a localised mod. But at the end of the day it's your call. Looks like the PE represents a leather pouch/pocket not a clear map holder to me though
Keith