Interesting and thanks for clarifying the rebound issue. Obviously, the rebound energy is dictated by the characteristics of the substrate. Stamping lead vs. hardened steel yields different rebound force. It could be by putting friction into the equation you can react the rebound energy. I have often just simply held the stamp between two fingers to lesson rebound. Applying friction may eliminate the need to apply differential force. Applying equal force, but adding variable friction may accomplish two things. It could overcome the rebound energy and also vary the force applied to individual letters. There are many creative ways to apply friction to a stamp. An example could be a pad pressing against the stamp with the friction adjusted by a bolt changing the length of a spring. To add some precision the spring could be calibrated and the bolt pitch would be selected for the appropriate change in length per revolution.