Bias cutting was used for lingerie throughout the 40's although War restrictions, 'Make Do and Mend' meant that the princess or 'A' line panel cut was the popular style because that style reduced the amount of fabric. As the fashion silhouette began to change - let's use 1947 as the date for the sake of argument - then so did undergarments and of course nightgowns.
Here is an original bias cut nightgown pattern from the 1940's illustrating 3 variations of the pattern. The version at the lower right looks very like the style we are discussing apart from the straps.
http://tinyurl.com/6zoy6l
I have seen all sorts of slightly misleading info out there about bias cutting such as;
'Bias cutting was promptly dropped as a waste of fabric.' (reference to the World War 11 period)
What is interesting is that the USA imported manufactured lingerie from the Philippines and Puerto Rico in the early 40's.In this next link, from a site I always recommend, is a good reference from 1942 on 'Modern Pattern Design' Lingerie. Specifically look at 'Bias Bra-top Slip.'
http://vintagesewing.info/1940s/42-mpd/mpd-10.html
I think it is true that a country's social and economic development, the impact of war particularly during the late 1930's and through the next decade would have influence on what was available in the way of lingerie and Nicole brings up a very relevant point.
BTW Nicole when you mention overlocking, re: the collection you bought, are the garments machine overlocked or hand overlocked?