Was researching an upside-down watermark on an Italian stamp from the 1930s, and Sassone specialized (my usual go-to on Italy) did not list anything for this issue.
Found this excellent resource, Catalogo Varietà del Regno d'Italia, freely available as a PDF from an Italian philatelist's website. Sharing this for anyone else who may find it of interest.
It covers varieties & errors in perforations, overprints, watermarks (e.g. letters, upside-down, sideways), setoff (decalco), etc. with rarity factors. Has many illustrations, and provides catalog numbers for 10 cataloguing systems. Covers the period from 1862-1942. In Italian naturally, but very easy to follow once the philatelic terms are figured out.https://www.francomoscadelli.it/cataloghi/