The plumber has arrived and his name is Mario (yes, I know...).
After an hour of trying things out, he decided he'll have to come back in the next few days with a probe and a special tool, because even he can't figure it out.

The positive side? At least the consolation that I would never have managed to do it on my own.

0

If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.bsd.cafe/users/stefano/statuses/116245838997957972 on your instance and quote it. (Note that quoting is not supported in Mastodon.)