On Monart, what is called the "button" pontil mark consists of a round blob of glass being put over the scar where the break occurred and then flattened and polished to make a place for the labels.
This wasn't done on some of the earlier Monart, and can depend on the construction of the actual piece whether or not it was practicable to put the blob on without damaging or compromising it.
Cellophane glass tends not to be opaque - hence the name Frank invented for it, because we are still clueless about its manufacturer, we just strongly suspect Czech.