It seems a `B.C.I` perfin. I have a couple of italian perfins, this one guess it`s common (I have cca. 8 of it). The problem with perfins is that they were considered `faults`, so collectors usually throw them. I found a big amount in a box, unsorted and separated from the rest of stamps. Just a few:

-edit: B.C.I seems to stand for Banca Commerciale Italiana (Italian Commercial Bank, founded 1894)
The `Patmos` overprint is from 1912, Italian occupation, Michel Cv 2005 MNH is 7 Euro (under Aegean Islans)