In determining what it may have been used on and what era, it is always helpful to consider the interpretation you get from other artifacts found in the same site you are digging. If you are in a park and are finding modern coins and pull tabs, then......However if the site you are searching has produced identifiable Civil War artifacts, then..... You have to look at the overall grouping of artifacts from our dig site because they all may support or reject an idea of what a questionable find may in fact be, given this context. Hope that makes sense, but it is often the "big picture" that we get from a site or even a pit, that enables us to make correct assumptions.And even that is not an exact science for there is always room for error.