"use the sleeves to make the rivets watertight" I guess it is not really accurate to say that the sleeve would make the rivet watertight but... The sleeves enclose the rivet. After the rivet has expanded water can only enter in that small area between sleeve and rivet. At least no water can enter the mast. The sleeve looks a bit like the metal cap at the back of a retractable pen. Check out the Murray catalog for a pic - I think I have seen them there.

Patrick