This one is likely known by the techies doing the adjustment of the valves themselves.
The used shims on a 400 do have the diameter of 33 mm. They are available on superformance for 7.27 GBP each. It's practical to have a lot of them in various thickness.
It turns out they seem to match with at least the following car models:
Lancia Beta 1600, 1800, 2000
Fiat: A lot !! For example Fiat Uno 1983-1995 or Fiat Punto
So if you are in need of a bunch against reduced prices I would say try the mentioned alternatives.
I will update the "compatibility part" section as well. For the readers who hooked on later: