This hook can be attached to a rear derailleur, not all derailleurs are tested, just a few. It's specifically designed for the short cage Shimano Zee and Saint derailleurs and 11-32t cassettes or smaller. The length can be adjusted over the entire length. 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 be used in conjunction with some kind of 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.
Length: max effective length 90mm
Material: stainless steel 304, 4mm thick, scotch Brite finish with polished half ball shape ends
Additional harware included: Stainless steel M5x20 Hexagon Socket head cap screw to replace the original lower pulley bolt
Weight: 23 grams