[veltro]

These questions are like debating on angels' sex... the same as the "Bf 109" vs "Me 109" ever popular controversy.

The main thing to know is that such designations were often used indifferently (even officially) and what is their origin.

SM stands for Savoia Marchetti, but S could stand also for SIAI, which was the name of the firm building those planes in the '40s.

CR (or Cr as often used) stands for Celestino Rosatelli, the designer of the series of biplanes; and G stands for Gabrielli, the other great designer for the FIAT firm.

Ro stands for Romeo (so I guess the first capital only letter should be correct).

MC stands for Mario Castoldi, the designer of the Macchi fighters, but also C only was widely used, as it happened for the Fiat G series.

That's it. Being no purist for such things I can only share these few infos. Of course there will be people more knowledgeable than me in this field, able to determine where, when and why different designations were used.

HTH,