To resolve this, you could look at the legal documents that define the boundary.

To find them, one source is the International Boundary Study on the Swiss/Italian border https://fall.fsulawrc.com/collection/LimitsinSeas/IBS012.pdf, which the US State Department published as part of a global series https://fall.fsulawrc.com/collection/LimitsinSeas/numericalibs.html in 1961. A better (more up-to-date and actually authoritative) source is fedlex.admin.ch, specifically SR 0.132 https://www.fedlex.admin.ch/de/cc/international-law/0.13#0.132 which lists Switzerland’s international boundary treaties currently in force.

For example, SR 0.132.454.221 https://www.fedlex.admin.ch/eli/cc/1976/2029_2029_2029/de is a treaty from 1976 where Switzerland and Italy agreed to modify their border along river Breggia. (Minor detail: Actually, this linked document is merely an informative, non-binding translation to German. For this particular treaty, the authoritative version is the Italian text; for every Swiss law, the SR page indicates which language version is authoritative). Unfortunately, the computer system for the “Sammlung der Räte” (SR) can’t publish graphics yet, only text. To see the map, you need to head over to the Swiss Federal Archives and retrieve the corresponding file in the “Amtliche Sammlung” (AS). [In computing terms, AS is basically a collection of changes to Swiss federal law, and SR is the current head revision of the main branch]. In German, the ID of the “changeset” starts with the letters “AS” (“RO” in French, “RU” in Italian), and this “changeset” is linked from the infobox in the SR page. In the example of River breggia, this is AS 1976 2029 https://www.fedlex.admin.ch/eli/oc/1976/2029_2029_2029/de, which links to a search page, ultimately (after some clicks) leading to this scanned map https://www.amtsdruckschriften.bar.admin.ch/viewOrigDoc/30001559.pdf?ID=30001559#page=31 .

Italy surely has a similar system for its legal documents, and in case of uncertainties, it would certainly make sense to look at both versions.

If anyone wants to spend time researching all this, it might be good to add it to the Wikipedia page on the Italy-Switzerland border https://en.wikipedia.org/wiki/Italy%E2%80%93Switzerland_border so it’s easier to find for others. The German version already has a section for the legal documents https://de.wikipedia.org/wiki/Grenze_zwischen_Italien_und_der_Schweiz#Rechtsakte, but it seems to be quite incomplete.

— Sascha