Probably a roll mark. If the hot glass glob is to large for the groove in the rolls. The marble will not spin correct. The excess glass will push out to a point. A round circle or crease will be formed in the hot glass, where the over size marble rolled around on the edge of the roll flight or top edge of the groove. The marble should spin in all different directions in each groove. If over size it will only spin in one direction and it makes a round crease or mark. Then the excess hot glass is pushed out to a point, or nub. If the glass is a little cold, that will also cause a wrong spin and a crease or roll mark. There are probably 100 things that can cause roll marks. Making 200-300 per minute, a few every ten minutes with roll marks is normal operation.