Pretty much pre-'73 Nortons had fiberglass tanks, with the filler cap hinged at the front. They were replaced by steel tanks in '73 (not sure about the hi-rider, as I've never seen a later one) with the cap hinged at the back. Something about US regulations.
My '06 Triumph Speed Triple has a plastic tank, but Triumph changed to a metal tank in '07, also something about US regulations. I know in Triumph's case, many '07s still came with plastic tanks, as they were allowed to exhaust stocks. The bikes used identical tanks in '06 and '07.
The theory is a metal tank is less likely to spill fuel in a crash, as it would dent rather than crack or split, and I understand this is the reason behind this rule. It supposedly applies to manufacturers of road-going machines over a certain volume of units sold. Triumph is supposed to have hit this magic number during 2006, and hence fell under the rule.
Some say a metal tank is more likely to catch fire, as it is more likely to spark while sliding down the road.