Films are not shot in 1,85:1. They are shot in full rame (at least back then) and then matted to the intended aspect ratio. For 1955 1,66:1 seems the likely intended ratio but 1,37:1 or 1,85:1 are not out of the question. The main thing is that you are not losing any picture information with a full frame presentation, though it's possible that the composition is not optimal. No doubt it was presented in a wide variety of aspect rations in cinemas around the world because widescreen was still a very new thing in 1955.
Thank you. So what you are saying then, is that the versions I'm seeing today are at the correct aspect ratio or at least how they were originally filmed and there are probably no others with a different aspect ratio available. Is this correct?
I had the impression it was framed for full frame. There is so much vertically to be seen in various scenes: Piazzo San Marco, the church (San Simeone Piccolo, I think) from the vaporetto (water bus), the views of the narrow alleyways, the view of the canals from the hotel patio, many others. I personally would have fought to keep the vertical content and not matte it off.
I watched this on my home cinema setup with the screen matted at 1:66:1 and the composition looked perfect to me, plenty of headroom, lines of perspective look good, etc.
1:66 seems to be the correct ratio. I watched the japanese Blu-ray and zoomed the image to get the 1:85 ratio. And it worked too. No heads or important elements were cut off. I hope there will be another BD in the correct ratio 1:66. But it looks good in full frame too.