[ajmac]

The adapter directs all of the pump discharge oil that reaches that point out of the block, the rear main feed issue is just a Flathead design quirk that they couldn't get around and so had to live with. The problem is that the rear main feed is drilled from a gallery closer to the pump discharge than the tapping on the top of the Bell housing.
Any restriction in flow up to the block mounted PRV doesn't matter at all as the original pump on all British Flatheads has no pressure relief, so the oil velocity will be higher through the restrictions but the volume flow will be maintained. Pressure will increase before the restriction to the limit of the pump gear clearance or mechanical failure.
I'll try and measure the dia of the gallery from the pump up to the top of the Bell Housing, it isn't much larger than the fitting extension tube, I think the clearance is just that, rather than a design intent associated with partial oil bleed off.

The waisting of the tube allows the horizontal Bell housing port to see post cooler / full flow filter oil pressure if you wanted to pop on a sensor. The cross sectional area is just less than half that of the fitting extension tube on my version. I will test all this out with some additional sensors when I finally get the 24 stud in my Loyd.