Good chance its metric. Just take the amp into a hardware store with ya and find a screw that fits. Or if you have a matching screw from another terminal just take that in with you and compare. The threads from the new screw should mate up with your original screw perfectly.
i figured its prob metric after i started ****ing around with the standard set, but i guess i could take it into the hardware store, might be easier than trying to match a screw to a screw