Swivel Link Assembly, Handbrake, this is the handbrake compensator link, it comes with the bronze bush 55780 installed & reamed ready for bolt 55781
It bolts to the axle at the one end & attaches via a cable abutment block at the other end
The differing part numbers are 71833 from the DB2 to MkII parts book, the 71883 from the MkIII parts book, same part, just a typo