The Club Modellismo Plastico Ravenna reports the S-84 as "SM-84 civile" meaning a commercial version of the SM-84 bomber that is incorrect.
Since the S-84 was designed by since like a sort of DC-2 (among other Italian types of that times like the FIAT G-18) while the SM-84 was obtained as evolution of the SM-79 bomber, especially developed as torpedo carrier for anti-ship warfare.
Curiously, internal layout picture is closer to SM-79, then to actual S.84 airliner's photo - compare, please, fuselage shape and vertical empennage.
They are completelly different. The only thing they have common is the name. It seems that the tri-motor bomber supposed to be designated SM.89, maybe to show its lineage to S.79, but maybe by mistake, maybe from other reason the final designation was SM.84. this didn't matter because a) the liner was properly named "S".84 (change from S to SM happened in the late '30s) and b) because it didnt really matter, since the liner was alread dead project.Thanks everybody for this interesting indormation. I have trouble understanding the background of the S-84 (for the DC-2 civil liner) and the S-84 (née S-79 bomber). Are they different designs? Or are they related or similar? Has anybody pictures of these two design lines? Further, who was resposible for designation model names? Was it the director of Savoia Marchetti, or was it the Italian government?