In the mid 1980's three of Saudia's 747 300s were delivered with an aux tank in the fwd cargo. It was a bit primitive - just a big cylindrical tank holding 1946 US gallons secured to the cargo rails by what looked like massive turnbuckles. I was told that the purpose of this tank was to enable non stop Riyadh - Rio flights. It could not feed the engines,it had a transfer pump to enable the fuel to be transferred to the Centre wing tank when space was available. I don't think it had a quantity indicating system as the aircraft would be despatched with it full or empty.It had a major problem in as much as the vent system had some kind of float switch which would shut the refuel valve if there was fuel in the chamber. I spent time checking how much fuel would go in before it shut off on each aircraft - not very much! I do not think it was ever used operationally, if I remember correctly it brought max fuel load up to 173 thousand kilos. Were these three aircraft unique as I never came across another installation like this in spite of spending over 30 years maintaining 747's?