The countermarks on coins of Ptolemy IV (coins C) are the key to determining the time of the monetary reform. Although, as a group, the weight distribution of coins A and B (with coins of c.96 and 72g weights not present) shows that a monetary reform has occurred, the only coins that individually show that they have been transformed by the reform are coins C (i.e., they are all countermarked). If the date of application of the countermark could be determined, that date would be the same as the date of the reform.
The latest known countermarked coin that is in, or not in, the hoard determines a date after which the countermarking must have occurred, i.e., a terminus post quem. Any earlier coins that show a countermark do not indicate a date when later coins were countermarked. This method determines the earliest date possible for the countermarking (and therefore the reform). It has been extensively used and is introduced in Part 4 above.
The latest known coin (Svoronos 1375, weights c.20g) that has the cornucopia countermark is a coin of Ptolemy VI who became king in 180 BC.
This assignment to Ptolemy VI was made by Price through his study of the Saqqâra hoards. Noeske also placed these coins shortly after 180 BC into the period of co-regency of Ptolemy VI with his mother Cleopatra I who died in 176 BC.