I have a postcard from Italy which has printed on it (on the picture side):
Ok, “Greetings from Chiesanuova” is clear. But what does that cryptic stuff in parentheses mean?
Chiesanuova appears to actually be located in San Marino, an independent “microstate” within Italy. So, “s. m.” might be a reference to San Marino?
According to Wikipedia Chiesanuova is at an elevation of 450 m above sea level, but neighboring San Marino municipality is at 750, so I expect it means whatever is imaged on the card is at 673 m above sea level.
Here’s another card that confirms my guess:
https://picclick.it/ROCCA-DI-BOTTE-Aquila-Panorama-263465862598.html#&gid=1&pid=1
At least if you google Rocca-Di-Botte and check the elevation.
ETA: I have to admit though I read the OP and thought “I bet that is Italian for meters above sea level”, which helped a little in searching for an explanation.
Good call. 
I don’t know Italian at all, but putting “673 meters above sea level” into Google Translate gives me “673 metri sul livello del mare.” Just putting in “above sea level” gives “sopra il livello del mare.”
Since “sul” or “sopra” appear to be “above,” and “mare” is, of course “sea,” it follows that “s. m.” is “above sea (level).”
Ah, thanks, that looks like it. I actually originally made the same guess but then when I saw the Wikipedia statement that Chiesanuova is 450 m. above sea level I dismissed it. I didn’t consider that in a mountainous area like that there would be a variety of elevations in a small area.