This hook can be attached to a rear derailleur lower jockey wheel, not all derailleurs are tested, just a few. It's specifically designed for a Shimano Zee and Saint derailleur and 11-26t cassettes. The length can be 15mm adjusted by means of the slots creating an effective length between 100 and 115mm center to center. It's purpose is to take up the excess length when the Brompton with derailleur gears is folded. On Shimano Shadow derailleurs it's recommended to use some kind of aditional B tensioning feature to pull back the derailleur when the bike is folded. Classic non Shadow derailleurs already have B tension with a build in spring.
Note: I cannot give aditional tech support to get you custom derailleur Brompton working and folding properly. It can be challenging and therefore you have to be experienced and inventive.
Length: Center to center distance 105-115mm
Material: laser cut stainless steel 304 with stone tumbled finish
Additional harware included: Stainless steel M5x25 Hexagon Socket head cap screw with Nylock nut to replace the original lower pulley bolt
Weight: 59 grams