I don't have a MP but I have brand of hub that came with those rings as well. They are for fitting the disc to the hub to space it away from the hub depending on what brakes you have as some calipers may rub on the hub.
Here are a couple of pictures showing the rings that were supplied with my Magic Pie:
They are the correct size to fit on the freewheel boss and are used to space the free wheel further from the hub when there is not enough clearance.
One should be enough, and is easily fitted by simply placing it on the threaded boss before the freewheel is screwed on.
If one shim is not enough, It might be wise to consider using a different freewheel instead of additional shims. Fitting more shims would further reduce the amount of thread available for fixing the freewheel, making it more prone to stripping the soft aluminium threads on the hub.
The shim takes up some of the threads visible in the picture on the right, which was taken before the freewheel had been fully screwed on:
They
should not be used for spacing a
bolt on brake disc as used on the Magic Pie, as the brake disc would not have a suitable flat surface area to be tightened against.
If you need to space the brake disc on a Magic Pie in order to give a little bit more clearance for the brake calliper, make sure you use identical washers (or sets of washers) on each of the six fixing bolts between the disc and the hub.If you're using the fixing holes nearer to the axle, you may need to file or grind a flat on each of the washers to ensure they clear the disc locating lip
(the sticking out bit) on the hub.
However, as Muzza has correctly stated, they
can be used on other hubs to space a
screw on brake disc adapter like this:
I hope that has made things a little bit clearer for everyone.
Alan