Cividale in Mantova is called Cividale Mantovano. It is a 'frazione,' i.e., an administrative subdivision of Rivarolo Mantovano. These frazione were created in the 1920s and 1930s and, usually, are villages and hamlets surrounding the main town of a community.
Cividale in Udine province, is called Cividale del Friuli. It is referred to as 'Cividale.'
Belluno's old name is Cividale del Belluno. Like Cividale del Friuli, its name refers to the old Roman 'civitas.'
There is, however, a frazione of Mirandola in Modena province.
The most likely candidates are the one in Modena if it had a post office at all, or the one in Udine province (Friuli).
My money would be on Cividale del Friuli. Google search shows a Lombard-Venetian stamp cancelled 'Cividale.' Unfortunately, the link shows two stamps on
ebay of which the one shown in Google search has a cancellation for Venezia.