I did some further researches and, as I said, sources don't tell always the same.
I confirm that the only photo is that one (by the way, taken in front of Fiat Lingotto factory).
The most reliable books that I know, "Gli autoveicoli logistici e tattici del Regio Esercito italiano fino al 1943" ("Italian Royal Army's logistical and tactical motor-vehicles until 1943", that actually covers also non-motor vehicles and up to mid 1945) says that it's not clear but probably it was just a prototype trying to adapt Opel Blitz wheels and Lancia "Einheits" cabins on Fiat 626 chassis, for the reasons told by Bill.
According to the Army (which, it worhts to say for who doesn't know in Detail Italian WWII history, passed in September 1943 from the former Royal Army to the new Republican Army when the King moved on the side of the Allies) records, only some hundreds of Fiat 626 have actually been built by Fiat in 1944/45. But, as told, the picture is not clear, because the new Republican Army - still allied with the Germans - was not very well organized and many documents got lost: also, Fiat was a supplier of Wermacht in Italy too, and again documents are not all survived.
I'm asking, to be honest, because I'm currently writing a book about Carrozzeria Scioneri and they started in late 1943 making "wooden truck cabins for Fiat". I'm trying to understand if they actually made only some plywood 626 cabins (that wasn't the Einheits but similar to the metal one, with curved panels) or if they have ever been involved also in the Einheits cabins production.
But, unfortunately, no records - once again - survived after the war and this early period is quite obscure, apart from interwievs released decades later by Mr. Scioneri's son.