Countermarks, on
Julio-Claudian coinage, were
applied to coins that
had lost sufficient
weight and definition so as to raise concerns about acceptability. So almost all
countermarked specimens are lower "grade" in collecting terms. As the
countermarked coins are relatively less common than the unmarked coins with simikar heavy wear, the
countermark is a "plus" in terms of collecting. Of course, some
countermarked coins were immediately lost and avoided further wear and retain crisp
countermarks, and some carried out their function for decades to come, and were often much more worn, re-marked and re-valued. So a crisp
countermark on a worn specimen is a collecting plus.
That being said, there are both ancient and modern
fake countermarks. It is usually a rudimentary punch mark, easily imitated. The leading article is
still, I believe, TV Buttrey, in the
ANS Museum Notes, 1970, available online. It is at least a
good place to start.