I've got a Besson International (Kanstul built). The 3rd slide is heavier. It's also longer than the 1st or 2nd slide which probably accounts for the weight difference.
Kidding ya, Van. Honestly the International is a pretty light-weight horn, unbraced tuning slide. I wouldn't use it to prove OR disprove your implied theory that Kanstul's third slides are heavier than other makers'.