Hi Mike,
Ah, Acorn nut eh - I was wondering what they were called. The only problem is the matching bolt has exactly the same head..... And the body section of the bolt is only a fraction of the normal thickness of what you would expect for that sized head. To me, it's a very unusual nut & bolt combination.
Maybe a solution for the bolt would be to put one on the lathe, turn the body down, then rethread it. The hole in the nut then would have to be filled, re-drilled and rethreaded.....

Or just get someone to turn up a few from scratch.
I've left the ones I have at the plating shop. I might get one back and take a close-up pic to show how they look.
Re the drain cup. From what I can see in the pic, it looks different to the black plastic E61 item. I don't have one, so not sure if the original is metal or plastic - I'm guessing metal by the colour and age of the machine - but who knows..
cheers, Stuart.