Most people interested in medieval armour think that the word sallet is first attested in the inventory of the Gonzaga family armoury after the death of Francesco I Gonzaga in 1407:
#23: Vna cellata coperta velluto carmesi pilloso cum certis dindinellis racamatis viridis “One sallet covered with plush crimson velvet with certain green embroidered fringes”
#25: Quadraginta tres cellate ferri “43 sallets of iron”
#124: Triginta stufe a celata “30 coverings for sallets”
That is the oldest reference in Claude Blair, and his book in 1958 was the last book on European armour by someone who spent a lot of time reading medieval documents in the original. In fact, there are another group of references from this time which have not yet been brought into the debate. These are in the Archivio Datini di Prato in Tuscany and were published by Luciana Frangioni in her article “Bacinetti e altre difese della testa nella documentazione di un’azienda mercantile, 1366-1410.” She copied and printed all of the references to armour for the head in this archive, and now I have copied them for you.