The orange cap is labeled "oil"...it is difficult, if not impossible, to believe that oil is not supposed to be inserted into said hole - though these are Morgan's we're talking about.
I suspect the right answer is to fill via the orange cap, but only to the level of the hole above the drain hole...
For my next flush, I'll be removing both the drain bolt and the bolt above it., I'll then replace the drain bolt and fill via the orange cap until oil leaks out the hole above the drain hole. I'll then replace all bolts and drive. If the car is still quiet, I'll leave things as they are, but if the rattle returns, I will then fill to the orange cap.
As an aside, the orange cap looks like a proper plug - not a breather, it has a washer...
