This socket removes slotted nut UA2756 that retains the pulley hub on the front of the crankshaft.  It is CNC made from Heat-Treated Chromoly.
This fits all V8 6.25/6.75L Rolls engines i.e., V8 Clouds, Shadows/derivatives and Spirit/Spurs until around chassis SC***09955.  Chassis' after this have a regular hex nut. 
 
This spanner also fits nut AJR 68 on the Austin Princess Vanden Plas equipped with the RR FB60 engine.  The BMC part number for this socket is 18G1009.