This stamp, and indeed the series from which it is taken, are given full catalogue status in Sassone, the main Italian catalogue. They are marked as "Non emessi" (not issued) but the catalogue then goes on to explain the circumstances, and point out that 11,000 sets were sold to collectors. Here is the note as it appears in the catalogue and the translation from Google Translate.
In seguito alla caduta del facismo questifrancobolli no venero consegnati aglisportelli per la vendita ma furono in un primo tempo messi da parte e, successivamente, soprastampati per commemorare la data del 28 luglio (serie seguente). Circa 11,000 serie complete vennero perņ lasciate senza soprastampa e furono cedute, tre anni dopo, per collezione.
Following the fall of facism these stamps were not delivered to the branches for sale but were initially set aside and subsequently overprinted to commemorate the date of July 28 (see the following series). However, around 11,000 complete sets were left without overprinting and were sold three years later to collectors.
It would appear the "selling to collectors" was sufficient for the editors to include it in the catalogue. The stamp is priced only for mint copies but I have shown below a used one; whether the stamp saw legitimate use is doubtful, although possible in light of the story above. The catalogue prices for mint copies are fairly low.
The Bolaffi and CEI catalogues do the same as Sassone, giving it full status and a "Not issued" note.