I think I know the ones you mean, which were used, AFAIK, only on the P40 series, as belly tanks. These were similar in shape to the later, 75 gal 'teardrop' tank, but I'd read that they were more expensive to manufacture, and later types of aircraft such as the P51 and P47 utilised the 75 gal tank, as well as the metal, and the 'paper' 108 gal tank, among others.