I'll post this for posterity in case it helps anyone. I haven't been able to find a definitive answer on "ad", but I had to research the related abbreviations "n." and "nn." today. These are Swiss legal abbreviations which co-occur very frequently with "ad" in quotations from published commentaries on Swiss law.

When I searched for "n.", I found the answer in a link which I'll post below, which contains similar references like this:

Jean-François PERRIN, Commentaire romand, N. 8 ad art. 289 CC

Elsewhere in the same link, very helpfully, N in similar contexts is written out in full:

Thomas KOLLER, Basler Kommentar, note 17 ad art. 328/329

Antoine EIGENMANN, Commentaire romand, Code civil I, notes 1ss
ad articles 328/329 et les références citées ; [...]
Thomas KOLLER, Basler Kommentar, Zivilgesetzbuch, 3e ed., notes 6ss ad art. 328/329
https://www.jura.ch/Htdocs/Files/Justice/1_Instances_judicia...

So n. is note, and nn. is notes. This Swiss example also confirms it:

GEISER Thomas, op. cit., notes ad art. 1, pp. 39 ss.
https://www.files.ethz.ch/isn/155938/Can you get sued in Swi...

I haven't found a definitive answer on ad, and don't need to as it isn't in my text (so far!), but I would have thought that "note(s) ad" would mean "note(s) on". This would make perfect sense when appearing just before "article(s)" in a legal commentary - notes on articles of a law/code. So I suspect that's what "ad" means here in this specifically Swiss context.