Metalli ?
Bingo!
I had no idea there were so many Carrozzieri beginning with the letter 'M' but I think nearly all of them must have been mentioned!
I did ask when it was from but I think now that you've found the coachbuilder we'll call it solved and give you your point; well done.
Carrozzeria Metalli were a very small Rome-based coachbuilder active in the late 1940s but very little seems to be known about them. This Fiat 500 was built in 1949 and here's the original picture, which I believe shows it in the 1949 Pincio Concours d'Elegance, where it was a prize-winner: