Dating & deciding shape of ladies straw trilby? hat

I have seen some hats like this in 40's/50's adverts but am very unsure when this dates from as there is no label and it has this fold mark all around the crown but it doesn't want to stay folded down or dented there and I'm not sure if it's supposed to be dented or not? It's a bit mishapen from storage so may not be in the original shape any more! Can you help with either dating or proper shape?