I think it was true capacity. I remember a RAF Mustang pilot telling me that the tank held 108 gallons, allowing for airspace for fuel pressurisation, although the internal VOLUME of the tank was 110 gallons. Sounds logical, if you consider that the physical space required for 2 gallons of liquid, across the top area of the tank, would not be much on this cigar-shaped 'paper' (and also some of steel) tank.
BTW, the 75 gallon, metal 'tear drop' tank was still used up until the end of the war in Europe by a number of Fighter Groups.
Terry.