This type of net was used during WW2, but there is also a net which is very similar made by the Dutch post war for their M53. If the size is 16"x 16" then it's got a good chance of being WW2, if 18"x 18" could be WW2 or post.
Should be a WW2 one, the 16" nets fitted the Mk2 & all the other British helmets until the arrival of the Mk3 which was bigger.The Mk3 gave overall better protection by 12% than the Mk2, against horizontal missiles by 15%, & vertical missiles by 11%. So in 43/44 the nets had to be bigger, which made them also fit the US M1. I think the smaller 16" nets might have been discontinued about then but I'm not 100% sure. You can get an early woven net & the 16" knotted nets to fit an Mk3 but it's a bit of a struggle.