The seat post bung from Brompton is a rubber bung which fits into the bottom of the steel seat pillars / posts stopping the seatpost clunking on the ground when the seat post is lowered / put down It also helps to prevent the bike rolling when Eazy Wheels are fitted and you don't want it to! (e.g. On a train or slight incline) It's also useful if you often park on tile / wooden floors as it will stop your seatpost chipping / scratching the floor. The black bung is for use with steel and (no longer available) titanium seat pillars The easiest way to fit the rubber bung to the seat pillar is to have the bicycle folded, lying on its side, or inverted / upside down Note: the rubber bung may be a tighter fit in (no longer available) titanium seat pillars and may require twisting to get it in up to the shoulder. Here's a very short video we made on fitting the bung