The center represents various periods throughout history and the future and has numerical significance with the markings contained within certain angular ratios.
The plate is interesting because it is one of the few brass artifacts found from this ancient civilization and this indicates part of the plates special purpose as a tool. It is part of an ancient timekeeping mechanism and allows you to interpret the data in that hidden sepulchre.
Perhaps another artifact nearby has clues to the location of this ancient mechanism.
Gold piece value: 80 gp
Gold piece value to a collector: 1000+gp