Hi Peter,
The spacer was introduced when the full rear chaincase became an option , I think in 56? The position of the rear case is dependant on where the clutch scroll fits into the sliding plate. There were /are different depth tapers on the clutch shaft adaptors..
It can be a PITA to build a bike from bits and get this right
Recently when going about fitting an electric start kit to my SR, the instructions said to check with a straight edge along the crankcase/primary joint face to see if it is parallel to the gearbox plate/ frame mounting...
A very small difference here causes a much bigger distance between engine and clutch sprockets
John