Jaime Kopchinski replied to an email I sent to his website, a very nice guy.
He suggested another supplier for the fuel return valve and I bought it this morning.
In a couple of days I should have a new reproduction valve and a spare diaphragm.
Then I will find out if the car actually revved without stumbling, or if it was my imagination.

https://www.mbclassics.de/carburator-nozzle-M108114121130180