Hi , while having a look at some of the unresolved questions I believe i have the answer to this one which i hope will be of interrest , I have seen on many occasions pontil marks in the bowls of 17th/18th/and 19thc glass ,it is generally thought that the reason for this is that if after completion of the whole item and removed from the punty attached to the foot the gaffer was in some way dissatisfied with the foot ,or possibly parts of a complicated stem formation as in some 19thc Venetian glasses ,it may have developed some cooling fissures in the foot , or maybe just a little out of the round , in this case the simplest method of dealing with it was to attach a punty to the inside of the bowl then the foot could be reworked and re heated till the desired result was achieved,The pontil in the bowl has also been noted in glasses which have some hot fired decoration applied , IE enameling as seen on Beilby glasses , and has been suggested that in these types after firing of the decoration the foot may have slumped a little in the kiln so the punty is applied to the bowl and again the foot is re worked till good.
cheers ,
Peter