J.P. 12" Super, Rear roller.

Hello. 

In the process of attempting the restoration of an early JP Super 12" I found the annular gear in the speed multiplier to have cracked through the centre hole! I have subsequently found a later doner machine only to discover the rear roller is very different but will still fit the original machine. The outer roller sections are formed steel and riveted to cast hubs and the centre sections internal dimensions are deeper to allow for majorly changed annular gear!   

My Question is does anyone know was the changes in design and construction just to reduce weight or was it due to reasons of mechanical failure as in my original machine?    

Early type shown on right Thanks Clive.