I think the screw is simply a blanking plug.
The problem with oil to the rockers seems to be the only thing that people know about Beagles!
The issue lies with the fact that the rocker boxes are sealed so pressure in them resists the flow of the oil mist up to them. Simple breathers of some description are likely to solve the problem (if it is one). To prove this to yourself, leave the rocker 'lids' slightly open when you get the engine running.
Have fun, its an interesting machine.
PS when it comes to timing valves etc remember the engine runs 'backwards'. Forgetting this wasted half a day of my life