Lug-to-lug is the tip-to-tip length of the case, from the end of one lug to the end of the other. It tells you more about how a watch sits than the diameter alone, because the lugs are what meet the wrist past the case. A small case can still wear long if the lugs stretch.
Put the watch on the wrist and look at whether the lugs hang past the flat of the wrist. End links, the pieces that join a bracelet to the case, change how that length feels. A gap you can rock with a fingernail is a fit question, not a claim about who made the part.
Diameter notes stay with the Fit index. This page is only the lug-to-lug measurement.
A library page is written by Watch Notes. A photo post is one person’s notes. Neither authenticates a watch or speaks for the name on the dial.