Could those slots be for bending different gauges of sheet metal
or bezel wire?
How you use it is up to you. But it may help to understand that
these riveting stakes were not originally designed and made for
jewelers. They are a watchmakers tool that some jewelers have found
useful. The holes and slots and all were originally designed for
doing some of the same sorts of things that watchmakers staking tools
do, as well as for some types of riveting or peening, etc. The slots
let you do things like resting the flat face of a gear on a flat
surface while the shaft through it has somewhere to go, or the shaft
can lay flat on the stake while a gear has that slot to swing into...
I'm not a watchmaker, so I can't fill in all the details. But the key
is that it's not really designed as a jewelers riveting tool. It just
happens to be useful for that as well. In use, the side of a rivet
that has not yet been peened over, goes down into a hole or slot, as
best fits, so it's not deformed or pushed back into the joint when
you peen over the side facing up. The stake supports the metal, and
peening over the rivet helps tighten the whole assembly. But there
are a lot more openings, and slots, and the like, in the watchmakers
uses, than are normally needed for typical jewelers riveting needs.