Since its unification the government has had to reorganize the local administrations. So the country was then divided into 4 administrative levels:
regions, provinces, circondari (districts) and mandamenti (departments)
by means of the so called “Ratazzi Law” of October 1859.
So a new municipal and provincial organization was created.
This organization was suppressed in 1927 but it is necessary to bear this information in mind because many records such as the lists of recruits were organized per districts and sometimes the documents of one district in one province are included in the state records of another province.
You may get a list of the historic Italian circondari in Wikipedia.