Used on L (with mudguard) and R (with mudguard and rack) model Bromptons

When your Brompton bike is folded, this hook hooks the front fork to the rear frame to stop it unfolding when you pick it up. (in combination with the seatpost being in the down position)

One of the "essential" Brompton spare parts - without this your Brompton won't stay folded properly

It works in conjunction with the lower stop disk and seat post (when the seat post is not in the down position the fold is not locked and picking up the bike will cause it to unfold)