To set copper rivets, first punch a hole in the leather to slip the post through. Put the base of post on a metal anvil or solid metal surface. Put the washer on top of the post and take the part of the copper rivet setter that has the hole in it to hammer the washer down over the post tight against the leather. Snip the rivet post about 1/8 inch above the washer with plier cutters. Then take the rivet setter part that has the small concave indentation and mushroom a smooth post edge over the washer. The concave shape of this copper rivet setter is less deep than the C.S. Osborne copper rivet setter for rounding the post end which results in a lower profile look. If you wish, you can take a metal hammer and slightly flatten the top of the post.
Make sure the copper rivet setter is for the same thickness of copper rivets. For example a #9 rivet setter is used for #9 copper rivets. The copper rivet setter has in one end a hole for forcing the washer down and a concave section for mushrooming the post.
seems to work ok
have not used the other