The earliest 'scuba' tanks were converted WWII aviator's oxygen tanks of about 40 c.f., fitted with a standard scuba valve. US Divers first started selling tanks designed especially for SCUBA in about 1951. They were rated at 2150 psi and had a concave bottom so as to stand on their own. These tanks were usually stamped 'RENE' - he was the first to sell them from his sporting goods shop in southern California. Some earlier scuba tanks came from Canada and France, but they are rarely seen in this country today.