stop going to that site they apparently know little lol lol
looks more like a musket thumb plate escutcheon plate . i'll see if i can find a couple to compare it to . yeah its definately a gun part heres a few examples , now finding the exact musket it went to is the question.
hopefully heres a couple of closeups , these thumb plates came in many different shapes , some were plain and some were ornate and some were even hand etched or engraved.