After some investigations I have a better clue about that metalastic element. On Earles fork BMWs there is a horizontal plate welded to the upper frame that supports the bottom of the metalastic with two M 6 screws. So you´d have to provide bracketry on the Vincent to this purpose, plus a bracket for the pivot at the front. I´d think you can bolt a bracket to the two tank mounting screws for the front pivot and some more bracket for the metalastic at the rear. Maybe using the sidecar mounting tube, or when having a rear carrier frame on the bike use that for a cross tube between sides at the suitable place for the bottom plate to fix the metalastic. But then I can imagine you may have any type of Pagusa rubber seat, tension spring, compression spring, or metalastic, and weld a bracket on the rear frame for support.
I still need some more months to arrive at this stage for posting my setup, but anyway I´ll have a rubber seat for sure. Our prewar Guzzi had the Lycette type saddle initially but I binned that for the rubber type for maximum comfort. Before it was like :
Rolling, rolling, rolling,
oh Lord. my ass is swollen . . . .
Vic
BMW seat
BMW saddle
PDF saddle, scroll down page 62 for picture
Sorry, could not add pictures from links, copy protection ?