Found it at a home site probably from 1800 to 1900 for a rough range. could be much older though. The piece is brass. Has a square hole punched into it. the number " 1 " on the ruler is in inches. thanks for looking
Kind of small for a jar lid-- I'm thinking probably the front piece of a brass button. The square hole might not be from original manufacture, probably later damage.
lab rat : I think it may be to big for a button. The picture has a big glare but the item is almost 2 inches in diameter. It would be a cool button though. I agree with you about the hold.