Has anyone ever seen or know what this thing is or is supposed to do?

It was connected on the Right hand side of Engine- between fuel rail and the RHS regulator.
It 240mm x 32mm, brass- weighty.. and rattles..

 

It's probably full of lead pellets, someone's attempt at a lead substitute when fuel switched to unleaded...

 

Well.. that would explain weight and the rattle!

 

Thanks Cambo,
turns out you are correct. Got it apart. - Lead Pellets in steel cages and a pair of magnets on the exit !

 

Somebody had zero knowledge of chemistry, since lead in fuel wasn't metallic lead. The V12 doesn't need lead anyway, it already has hardened valve seats.

 

Yes, zero chance to produce significant amounts of lead tetra-ethyl, which is the useful stuff, by just flowing gas through a lead pebbles bed.
There are other devices that are sometimes mounted on the fuel circuit of classic cars, e.g. Eurocat containing tin based pebbles, which are claimed to act as oxygenation catalysts for fuel burn and/or to load some tin compounds in the fuel with the aim to better protect valves seats against wear when not using leaded fuel... Not very conclusive either