Replacement glass for Cartier Santos

Was hoping someone could recommend where I could source replacement glass for the Mrs' bi-colour Santos automatic, she carelessly rattled off a wall!

Unfortunately I don't have a serial number as it's a well-worn model (I believe it's from the mid-90s) and has been polished off. Did a quick search online and found a place called Timebymail, they've got various sizings in the square format I'm guessing I could narrow it down by measuring the bezel?

(FWIW the mrs was a jeweller in a past life, she'll be doing the replacement herself. I don't have the capability to open the case myself for identification purposes and it's a surprise gift for her, hence my needing some input).

Santos square bezels come off the front and if the seal hasn't been replaced for a while you then it will be rock hard and need replacing.
If the numbers are worn, the only way to know for sure is to remove the bezel and measure the glass.