Hi Bill, I imagine the PRV would open with higher revs ( not had bike running yet ) but the oil still has to flow from the PRV up to the horizontal oil tunnel and the across to meet the tunnel going up to the cam shaft. I mentioned this episode as I believed the A10 c/cases are same as Ariel??. Where the two oil tunnels meet ( cross drilling ) MUST have an alloy plug fitted at this junction or oil would simply p**s out into the timing cover and not reach the cam trough. The plug isn't readily visible as it is ground flat (and merges with the crank case material) to accommodate the back face of the pinion .
I should have said, to make it clear that when the tunnels are factory drilled the drill has to pass right through the tunnel in both directions. The short extended end of the defunct part of the tunnel is then plugged. If you look carefully one can see the plug has been ground flush.