Isn't there any law saying that "if no one claims it for N years, the state takes it" / "if someone else uses a place for N years and no one else claims it, that person can claim it?" [1]
However, Italy also abolished all noble titles with the fall of the monarchy in 1946 -- I would have also expected that fiefs and any associated claims had been extinguished at the same time...
But this too was already fraudulently abused. Generally, a poor rule of law complicates everything.
(just as aside, communist land "reforms" are most visible but there were others, based on nationality - taken from Jews, Hungarians, Germans)