Andre the plug sticking out is the locating dowel for the oil pump drive spindle. It has a chisel shaped end which sits in a groove in the spindle. It's obviously been semi pulled out and is not sitting in it's correct position. If it's stuck? Pull it out by screwing a 1/4 x 26tpi bolt into it. Clean it up and put it back properly. If it sits down too far into it's hole, place a small washer on top of it to make up the gap.
I'm not sure about the timing cover...M20 covers are available which do not have the 'lump' in the casting (That was machined out for the breather assembly when the cover was fitted to an OHV engine...)...
It is clear that at some point a common casting was used and machined (or not) as required...but in which year was that introduced?...I don't know personally but I'd be happy for someone to tell me!.. ...Ian
There is a picture I think of a WB30 which shows the breather fitted using this casting, so the molds did probably exist early in the war, then as production of the WM20 was increased and old molds wore out I can see no reason why this wouldn't have been used. It did waste a small amount of alloy but as its not aircraft quality it didn't make much difference.