Hi Miranda, The one on the right is ribbed on the interior of the foot the same as on the exterior...... Kind of like it was a long coil that was laid into a mold one turn at a time..... if that makes any sense.... Both have a fire polished rim and both have a rough pontil mark up inside the foot. The one on the left is a little textured on the interior, but more just irregularity in the glass from production than any pattern that follows the trailings. The thickness of the glass in the bowl is quite different in the two. Hope this helps.....
I do not know if mine are that old for sure or not... Just what the old paperwork with them said when I bought them..... They are really fascinating pieces......