It seems the change to bronze was originally planned for 1858, but there were so many problems they had to continue with the copper series. That's why there are so many overdates for 1858 - they were pressing old dies into service to meet demand. 1859 may be a case of preparing dies ahead (in 1858) as they knew they still wouldn't be ready. Logic says the changeover was rescheduled for 1860, but they still weren't quite ready, so a small number of 1860/59 coppers were struck. Note that the issue of bronze dated 1860 is massively outnumbered by the 1861 - 1863 mintages, indicating that it may well have been quite late in 1860 that they finally began striking bronze? Otherwise you would expect an equally huge mintage for 1860, being the first year of bronze.