This is the distance piece that betweens the cams and the cambox back bearing.
It ensures the back bearing inner cage and rollers are sititng centrally in the outer bearing (which is located in the cambox rear cover).
Assembly Note: This distance piece has been copied from an original pattern - taken from a very original pre-war SOHC M30 (magnesium) racing cambox, which has not been apart for at least 50 years before I stripped it down - and as you see here our distance piece looks just the same as the original. However, you will also notice that the spacer has a slot to feed over the camshaft woodruff key, that retains the cams.
I am not sure why Norton's did this, as it is normal for the the spacer ring to be fitted between the cam and rear cambox bearing (i.e. before you get to the woodruff key), however what is most important is to ensure the cams are running as close to central under the rockers - so I assume that Norton's did this because there may have been some variations, whereby it was possible to fit this ring behind the cams if necessary - to ensure they are central to rockers. However, for initial assembly - I always follow the 1948 Spare Parts list exploded diagram and fit it initially between cam and back bearing.You should always check that the cams are acting on the rockers close to central and not fouling the other rocker.
This distance piece is @60 thou thick, which I find to normally be correct, but you should check yourself that this thickness locates the bearing in the correct position, as this can sometimes vary as there were many slight variations over the years. To help with this, place thick high melting point grease in the outer cage of the back bearing, then trial fit the back cover, you can then see from the marks in the grease if the inner race is in the correct position.
Note: This distance piece has nothing to do with setting the camshaft/vertical bevel gear mesh - for those shims, see Item 0328 and 0328B