I have a set of standard carbs 26mm Mikunis and have been trying to find a guide to rebuilding them. Can't find one so just tearing them apart

Anyhoo, I appear to be a bit missing. There is a plate which goes at the top and joins them all together, part number 16065-040 - part 42 in the diagram

Do I need this bit? can I make one? anyone got one? what's it for?

Richard