The standard tank was 90.9 litres as you rightly say so assuming that alls well below deck i.e. the gasket on the inspection hatch isnt shot and the fuels simply leeking out, then the tank is obviously bigger than standard.
The filler pipe will hold a few litres but that wont account for the difference between 90 and approx 120.
However, i once worked my tank out to be 30 gallons using a fuel pump and dipstick but some how i got it wrong?? and it was indeed only 20g. Having a bad numbers day i think!
Not sure if any special builds are out there but so many were built, i guess anythings possible. I have heard that for long distance work, the MoD used saddle bags which hung over the tubes.
Also, it may well have been converted some time ago and been done professionally, thus looking similar to the original quality.
Just a few ideas..........