Tiagra "Double" rear Derailleur. 265 grams.
This version of the Tiagra rear Derailleur features the short-length "SS" cage.
(See Capacity in Sheldon Brown's Bicycle Glossary if you have questions as to the capacity you need.
This is nominally a "10-speed" unit, but works fine with any number of speeds as long as the shifter matches the number of gears in back.
We recommend replacing the inner shift cable if you are changing the Derailleur.