hello Della - sorry, not a clue as to origin - know I've seen such things in the books somewhere, but can't for the moment remember where - quite likely it's Patricia's bulb book.
My only possibly worthwhile comment is that rather than an inverted pontil - seen for example on some pieces of Alum Bay glass - your indent is more likely to be a 'kick' - used to strengthen the piece I believe.
An sure somewhere is already delving through said book for you - fingers crossed.