Those 3 ellusive washers/spacers for the center vacuum tube.

Playing the Devils Advocate:
If I were in the situation of not having them - Would I assemble the carbs without them?
Although they are intended to avoid metal to metal contact I wouldn't loose riding time because of not having them.
Just remember: Without these spacers the main needle will now be lower (leaner mixture) !
So ......The main needles will need raising one groove (assuming you have the 5 groove style) to maintain the top-end mixture.
In the mean time I'd be sourcing them. The "Catch-22" : Might as well "make" them and be done with the issue to avoid having to re & re the carbs down the road when you do have them - "Home-made" or otherwise.

Why dissecting this if we hate academical experimentations, which I know you hate!
I offered a simple stock replacement solution. Even better - up to date material wise made in Japan, because it was made currently in Japan.

Then people wonder why their bikes don't run... DIY innovations are great, but sometimes stock is best. After all, negating the thought behind the original design of the time would be like trying to insult the original makers. The straight off the line KZ1300 I bet worked well. Keeping in mind all the modern times possibilities, personally, I just can't stray away much from considering the original design, which got tested relentlessly over many years before release.

They were put there by Mikuni for a very good reason so need to be there.

If not , they would not have put them there.

Pete F
UK

Was missing one of mine, and the PO (for whatever reason) replaced it with some gasket material. Which was in the process of falling apart.

Thankfully, z1300.de sells replacements (metal rather than the original plastic), so they are available. Even IF shipping costs more than the item itself.