Those caps make a strong case for 1.66:1! But, JUST to play devil's advocate here, are we sure that the original 1.37:1 caps represent full aperture and haven't already been cropped on all four sides? That would make it nearly impossible to get accurate readings if you further crop them from...
I'd love for that to be true, sir, but the new caps at the Criterion site are at 1.37:1. That doesn't bode well, does it?
https://www.criterion.com/films/368-summertime