Round balls are made to be used with a patch. That is, a bit of cloth wrapped around the ball to seal better in the rifling of the barrel. Therefore the balls are generally a tad bit smaller than the actual barrel caliber. As previously stated, that ridge appears to be caused by the two halves of the mold being out of alignment, probably due to a lot of use. The little bump you see along the seam is the sprue, left from the opening between the two halves where the lead is poured into the mold. When the ball is loaded, the sprue is always pointed up, and is usually somewhat flattened by the ball starter and then the ramrod. Given all this, I agree that this ball was probably a reject, destined for a recast.