Hi Michael,
Shims behind the cush drive are
sometimes required to obtain correct primary chain alignment
They need to be fitted between the main bearing spacer and the splined drive sleeve,
NOT between sprocket and drive sleeve!!!!
They should be a reasonably good fit on the crank, I'm kind of presuming that you were supplied with the shim pack sold to obtain correct crank end float????
These will have a bigger diamater hole in them and would be difficult to centre???
Firstly check with a good straight edge the sprocket alignment, if required then add shims to correct,
once you have decided the correct thickness needed its easy enough to either cut a correct sized shim from shimstock or to turn up a sleeve and part off suitable thickness shim/washers
On a similar issue I recently noticed that on examining several primary sprockets I have, there are different thickness or no shoulders where they sit against the drive sleeve????
(I know that B & M series BSA's can even have machined depressions at the back of the sprockets)
Also the drive sleeves can have different thickness shoulders between sprocket and crank spacer ????
The more I see the more confused I get
HTH
John