Hi Wilko,
The singles have a boss at the crank endas part of the inner case to space it out further than a twin case
but the twin case has a spacer to do much the same job
The single models have the chain 1/8 - 3/16ths further in than a twin
The reason for the cut out on the singles is to access the points on the magdyno
The outer covers are the same, thats why theres a thick section at the top of the outer case
HTH
John