Watch List notes:

Caracalla. AD 198-217. AR Denarius (19mm, 3.28 g, 1h). Rome mint. Struck AD 208. Laureate head right / VOLTA SOLVT DEC, COS III, Caracalla standing right, sacrificing from patera over tripod; before, victimarius raising axe to strike bull; behind, flute-player facing. RIC IV 205; RSC 684. VF, light porosity, minor edge splits. Rare reverse type.

This interesting reverse type refers to the vows completed by Caracalla for his first decade as Augustus, and his taking of additional vows. The sacrificing of a bull was a customary accompaniment when the vows were completed.