In the past with some of the more remote destinations in the pacific, under FAA regulations an "island reserve" fuel could be carried if no alternate was available. This island reserve fuel was two hours at normal all-engines cruise speed. for four engine aircraft.
This did not apply to twins because ETOPS was not then available.
So far as I know, this island reserve fuel requirement is still in the regulations. PanAmerican used this procedure quite a lot years ago with the B707.