Genuine Brompton derailleur chain tensioner assembly for 2- and 6-speed models.

This can be identified by the fact it has 2 identical looking black jockey wheels where the chain sits in a groove in the jockey wheel.

This works as a chain tensioner (keeping the chain tight when riding and folded) and as part of the Brompton designed derailleur mechanism - by having juckey wheels which can move from side to side to allow the chain to move between the two rear sprockets.