These small stone stamps were used for marking semi-solid sticks of eye-ointment (called "collyria") before they hardened. The edges of the stamp are engraved with abbreviated Latin inscriptions in reverse. When stamped into the ointment, the impressions could be read correctly. This example has the name of Titus Vindacius Ariovistus, probably the maker of the ointments, and names that would identify different types: 'nard-oil salve', 'green-salve', 'infallible salve' and 'frankincense salve'. The name 'Senior' cut into top surface may be that of a healer who used the stamp and ointments.
You can listen to Professor Helen King, Professor Emerita of Ancient Medicine, Open University, talking about oculist stamps here: