Definitely a brass rivet, used to hold two pieces of thick leather together. Unfortunately they are almost impossible to accurately date to a specific time-period, because they were in use from at least as far back back as the 1700s and are still being manufactured today. Most that we relic-diggers find are from horse-harnesses, but some were from luggage and some were from military cartridge-boxes. But as you'd guess, there's no way to tell a civilian-use rivet from a military-use rivet. Of course, the majority are from civilian-use horse equipment.