Just to add a bit more: the first few years of the changeover to bronze saw a helluva lot of repunching of date digits and legend letters. Bronze is a harder metal than copper, and the coins were thinner in size, so dies got worn relatively quickly. As well as that, the Mint is reported to have experienced a lot of problems caused by the change of metal, which the huge number of varieties (major, minor, and micro) in the first few years can attest to.