I am rebuilding the carbs on a 1971 BMW 3.0S (German spec.)

In my manual for the Zenith carb it is stated that a vent chimney MUST be installed on ALL 3.0L models.

And of course, I do not have this on my carbs, only a tower.

What is the function of this chimney? Does is have something to do with the shim thickness under the float level needle valve? (2500/2800 has 1 mm, and 3.0L has 2 mm) On my carbs there was a 1 mm shim when I opened it.

Here is the manual I've used, which is a lot better than the crappy Haynes manual with little details/specs/photos. :

My alternatives are:
1: Run the engine without the chimney, but with 1 mm shims
2: Run the engine without chimney, but with 2 mm shims
3: That I really have to install the bloody chimney mod kit, and spend lots of time trying to trace one of them down.

Come on all Zenith experts out there, please give me an answer on what to do!
(And please no answers containing the word "Weber"...:-)