The part you have is not Hamilton-Ball. Your pic's won't enlarge.
You are going to need a Hamilton-Ball donor to put the watch back to original condition and you are going to need 4 parts.
The regulator arm, the whip spring, the screw that goes into the side of the balance arm and the nut with pin end that goes under the regulator arm.
Maybe you got a good deal on the watch, but sometimes good deals are not so good.