Probably the definitive text (in English) on the Regia Aeronautica is that by Chris Dunning, 'Courage Alone'. And he shows the designations for the aircraft you are interested in as:
Savoia Marchetti: SM 79, SM 81 etc
Fiat: CR 32, CR 42, G 50
Meridionali: Ro 37, Ro 43
Macchi: MC 200, MC 202, MC 205
Yet even in this book there are varitions to the above, as in the text the above aircraft are referred to as above, while in the colour profiles they are shown as:
Savoia-Marchetti: S.79, S.81
Fiat: CR.32, CR.42, G.50
Meridionali: Ro.37, Ro.43
Macchi: C.200, C.202, C.205
So heaven only knows.
