Ron L wrote:So, theoretically, if you block off the hole in the crankcase you can use an Atlas timing cover on an early Commando (20M3) motor with no ill effect?

There's no actual need to block it off, if there's no corresponding hole in the timing cover. [Edit] Thinking about it, the gasket face of an Atlas or Dominator timing cover may not blank off the drilling in the Commando case, so as you said, you may have to blank off the hole in the case?

The 850 MkIII timing covers do not have the drilling either, as the MkIII models also vent their relief valve oil directly into the timing chest, however the MkIII T/S case still has the earlier drilling, which isn't blocked off.

Ron L wrote:I wonder what was their reasoning for adding the oilway? Was there an issue with excessive oil in the timing chest of the Atlas? If so then why go back with Mk3?

I don't know why they adopted that idea for the (pre-MkIII) Commando?

In theory I would guess that recirculating the OPRV oil back to the oil pump inlet allowed the scavenge to empty any excess oil from the sump more rapidly when the engine was started, instead of that oil being dumped straight into the timing chest where it would quickly drain to the sump and increase the oil level, but that's the only actual benefit I can think of?
I did hear the reason it was discontinued on the MkIIIs is because it was found that the OPRV could cause frothing of the recirculated oil, but I have no idea if that is actually true or not?