I have no idea what those numbers mean. Usually when buying pop (blind) rivits you get a range of thicknesses they will work for, but that won't work here (ie 6-6).

Since they come in 1/16" in dia encrements and 1/8" length increments it might be as follows:

6-6 is 6/16" (3/8") dia and 6/8" (3/4") long.
6-4 is 6/16" (3/8") dia and 4/8" (1/2") long.

Maybe someone else can shed some light on this.