Hi Tony
There is a large threaded alloy ring that secures to the hole with grub screws (part # 14 & 15 on the parts dia) and the filler then screws in to it. Although this parts dia is from the later '92 bike it is the same, there seems to have been 2 851 alloy tanks one like yours and one that used the same sort of mount as the older Ducatis (my 750 Sport has this type), I have the same type of alloy tank as yourself.
The alloy ring is the same as lots of other Ducati's - SS, ST, MON etc so should be pretty easy to pick up.
Might be an idea to download some part cats - most can be found in the Files section.
Johnny