This is the table of origin codes for Italian plates between 1905 and 1927. As you see, numbers were used instead of letters and the provinces were sorted alphabetically. I made them in red as they were printed on the plates.
| 1 Alessandria | 2 Ancona | 3 L'Aquila | 4 Arezzo |
| 5 Ascoli Piceno | 6 Avellino | 7 Bari | 8 Belluno |
| 9 Benevento | 10 Bergamo | 11 Bologna | 12 Brescia |
| 13 Cagliari | 14 Caltanissetta | 15 Campobasso | 16 Caserta |
| 17 Catania | 18 Catanzaro | 19 Chieti | 20 Como |
| 21 Cosenza | 22 Cremona | 23 Cuneo | 24 Ferrara |
| 25 Firenze | 26 Foggia | 27 Forlì | 28 Genova |
| 29 Agrigento | 30 Grosseto | 31 Lecce | 32 Livorno |
| 33 Lucca | 34 Macerata | 35 Mantova | 36 Massa Carrara |
| 37 Messina | 38 Milano | 39 Modena | 40 Napoli |
| 41 Novara | 42 Padova | 43 Palermo | 44 Parma |
| 45 Pavia | 46 Perugia | 47 Pesaro | 48 Piacenza |
| 49 Pisa | 50 Imperia | 51 Potenza | 52 Ravenna |
| 53 Reggio Calabria | 54 Reggio Emilia | 55 Roma | 56 Rovigo |
| 57 Salerno | 58 Sassari | 59 Siena | 60 Siracusa |
| 61 Sondrio | 62 Teramo | 63 Torino | 64 Trapani |
| 65 Treviso | 66 Udine | 67 Venezia | 68 Verona |
| 69 Vicenza | 70 Pola | 71 La Spezia | 72 Taranto |
| 73 Trento | 74 Trieste | 75 Zara | 76 Fiume |
As you can see, Agrigento (29) and Imperia (50) are not in the correct position if you sort the names alphabetically. The reason is that those cities changed their name: in 1905 Agrigento was called Girgenti and Imperia was Porto Maurizio. This makes sensible their position on the table.
Provinces with number from 70 and up were introduced in 1923 (Fiume in 1924) and this is why they are not in alphabetical order.
I want to thank Guglielmo Evangelista very much for his help and corrections!