I collected these for a decade or two some time back. They run from 1764-1781 in denominations of Polushka (1/4 Kopek), Denga (1/2 Kopek), and 1, 2, 5, 10 Kopek. All 1764's are quite rare -- they start getting common around 1769-1770.
@KSorbo, thanks for posting. That's one of the most spectacular coins I've seen in quite some time. World coins never cease to surprise!