My New Imp has a Dunlop rubber saddle. These are a moulded piece of rubber in the shape of the saddle, supported across the rear by a frame section and at the nose by a hinged assembly. These two frame sections are joined together (or more probably held apart) by a "V" shaped wire spring. Loops formed in the ends of the "V" bolt to the rear frame section, and conventional saddle springs bolt to plates close to them. It's a very comfy seat, and didn't leave me with black clothes as I expected it would.
Reproduction rubber tops are available in the UK. Mine came through a mate so I don't know the precise origin, but I believe it was a "small trials saddle".
cheers,
JFerg