This item is not used when a Water Pump is fitted.  (See item 172 for the special sprocket that is fitted in this case).

This Sprocket carries out the same function as simplex (single chain) as used on the 9 LM's which is our item 231.  

Duplex Chains are used on all the 6 Cylinder engines, the bigger 4 Cylinder Singer 12's and HRG's as they rotate at twice the Crankshaft speed and are generally very badly worn.