The die's image is of course reversed, so the numeral is incluse and the field is raised. As metal flow occurs over the edges at the raised/incluse border of a device such as the number, it molecularly acts as an abrasive. the steel die is much harder than copper, but under high pressure and tens of thousands of strike, erosion/deterioration occurs on the edge rounding it off, then it will appear as a raise area on the struck coin. That is why most often die deterioration looks like the original device ( same curve/shape). Most often the deterioration occurs in the direction of metal flow and most apparent where there is a sharp change of direction of the die surface ( as mentioned above). Not common on smooth transition from a flat area to raised, visa versa.
approximately 800,000 to 1,400,000 strikes per die with reverses usually lasting longer than obverses. Also, if you find letters or numbers "filled" in, then something broke off, i.e. die chip/crack/break.
