It's a good question that ive pondered from time to time

For A-series engine, the PCV valve screws into the intake plenum. On regular A12S (single-carb engine), this valve has a hose to the block breather tube. But on the Twin-Carb engines (A12T and A12GX) the hose connects to the valve cover tube

my guess is it was done for convenience, easier to route the hoses that way to side-draught manifold and air cleaner. In any case it looks neater

How did the Austins with twin SUs route the breather hoses? Perhaps whoever designed the twin Hitachis used a similar layout?

I also wonder about the block breather. It has a metal gauze/baffle insert to help separate oil from the crankcase vapor. Would that work when the vapor is pulled from the valve cover instead of the block breather? I should have had a closer look at the GX rocker cover, maybe it has that built-in. The single-carb valve cleaners do not have a breather gauze but only a simple metal baffle

Block Breather Mesh