It's called an absorber. Fitted to the transfer boxes running behind the 5-inline-cylinder engine (Td5) to low down vibrations.I've got one on my TD5 D2. Come to think of it, I believe it's on all TD5 Discos and Defenders.
IAI 100030 is the number of interest.
It's fitted by 3 special M10 studs: IYR100010 and nuts FY110046
Listed here: Just have a look . Have a look at #47, #46 and 48
It's a 5 cylinder thing. Hope you don't need it!