Dating from 1961 , the Minute Repeater ref. 6448 , powered by manual cal. 13''', was Vacheron Constantin 's last vintage minute repeater before the reintroduction of this complication only 30 years later with the caliber 1755 and reference 30010 (if I'm n...
13,75 lignes (= 31,02 mm) and that would be the diameter of the caliber. Obviously the measurement indications relative to this watch are not very accurate as they differ from one source to the other.
So it's a description of the type of caliber. And 13''' means 13 "lignes", which is a unit of measurement for watch movements (not a reference number). 1 Ligne=2.25583 mm (1 mm=0.4433 lignes). So 13''' (13 lignes) means that the movement is 29,33 mm in di...