It often became necessary to try to expand the life of a die which hadn't been completely used up. This often happened when the mintage demand resulted in a shorter run than anticipated, leaving many dies prepared for that denomination and year date unused or left over. To avoid discarding the dies, the mint engravers occasionally reworked the die designs and actually punched or carved a new date or design feature into the die to allow it to become usable once more.
This type of modification was well known in the years between 1795 and 1900 when such practices were acceptable and it was considered to be wise use of the materials at hand. This has resulted in many coins of quite a few denominations which are known to have multiple dates, where one date was almost obliterated by the over punching or re-engraving of a new date.
The hubbing process consist of steel hubs which are pressed against bars of steel in hydraulic presses to create the desired designs in that steel bar face. It invariably requires multiple pressings of the hub into the die to built the proper design with the correct depth and relief. However, when the hub is pressed into the die steel, the die steel hardens due to a molecular effect known as "work hardening". The die steel is therefore softened by an annealing process in which the steel is heated to a cherry red color. It is then permitted to cool very slowly, relaxing the molecules of the steel.
When cool, the soften die is brought back to the hubbing room where the hub is placed against the die and it is hydraulically pressed again. This operation would continue for another 3-5 times. On rare occasions, usually during the end of the year when production of dies for the next year has started, it has been possible for mix-ups to happen. A hub with one date is used to strike a die. That die is sent for annealing. When it comes back for another impression from the hub, it is accidentally struck by a hub with the next year's date on it. The die will then have two dates on it. A few of the most recognized examples of this is the 1918/7 D Buffalo Nickel, 1942/1 Mercury Dime, 1942/1 D Mercury Dime and the 1918/7 S Standing Liberty Quarter.