Glass door
Pete
This was a very common arrangement in electric cookers from towards the middle of the 1950s onwards. Normally if a cooker had a bottom hinged oven door it would have a glass window built in to it but if side hinged then all but the most BOL cookers has a secondary glass door - even my mothers Jackon from 1957 has a glass inner door. The idea was to allow cooking progress to be checked without all the heat rushing from the oven which I suppose it did to an extent, although they were by no meants tight sealing.
I think this was a British thing, and just on electric cookers as I recall, and began to die out from the early 1980s with the adoption of completely glass doors and also with the massive increase in sales of German ovens (Neff, Bosch) which had bottom hinged, and in the case of Neff at least, glass doors too.
Al