Hmm, size of seat posts got me checking.
Unfortunately, my new deluxe digital callipers too sophisticated for me to operate, so back to old mechanical ones.
753 less than 27.2, could be 27 or 26.8 and vaguely remember that tubing for French was slightly smaller than for UK. Know it's less than 27.2 because happen to have Dura Ace seat post that I know is 27.2 on my Gazelle 731 OS and the 753 is definitely smaller, so in the end my curiosity made me take the post out and there it was, Campagnolo 26.8 (also, discovered that after the winter we've had it was a good time to take the seat post out and re-grease ..).
Moving the callipers to the two 531 Pros and, voila, they are both smaller than the 753, so could be 26.4, but I can't be certain on the exact diameter.
You should now have a result, and my 753 won't have a seized seat tube so we should both be happy!