Rinaldi hammers come from the factory with two identical ‘flat faces’ which are quite rounded (convex). A rounded face will move steel more effectively, since the impact of the hammer is concentrated on a smaller area (especially when peening on a narrow-face anvil). However, it requires more accurate placement of the strikes on the edge of the blade. With a less-convex hammer face the impact is wider and it is easier to overlap individual strikes and achieve a more uniform result.
Years back, when I first received one of these hammers, I flattened one face to my liking. Most of the time I use the flatter face, however, for some particular applications I choose to work with the more-rounded face (for instance, drawing-out a thicker, neglected edge or some factory edges near the point).
To improve their versatility, I flattened and polished one face on all of the Rinaldi hammers I offer.
Please note, the special peening hammer is required only for free-hand peening on the anvil. You can use an ordinary hammer when peening with the jig, since you’re striking only the cap of the jig and not the edge of the blade directly.