Melanie, I think the '30s style gown is the right way around because of the train. You're right in that the buttons are usually down the back but occasionally you see them down the front.
Wedding gowns are the most conservative of styles and I would have agreed with Melanie's datings were it not for seeing hundreds of gowns along with the date they were worn - '30s style gowns were worn well into the '40s (also some were reused due to wartime fabric shortages etc) and the '40s styles crept into the '50s. Another factor is the location of where the gowns originated. As mentioned. WW2 impacted some areas a great deal and less for others. Eg, if the '40s gowns were from a place that had rationing or wartime restrictions, they're likely to be late '40s rather than early or mid. Another factor is the fabric, as some (eg silk) weren't available during the war in some places.