This chain tensioner is for Brompton folding bikes with 2 speed, 6 speed SRAM or 6 speed Sturmey Archer / BWR (Brompton Wide Ratio) gears.

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 jockey wheels which can move from side to side to allow the chain to move between the two rear sprockets

To make your chain tensioner last as longer - keep your chain clean and not over lubricated