Worth mentioning here, for those who may have an interest in the process and history of machine threading, that there's a fairly substantial write up on this subject in Charles Hajdamach's first book 'British Glass 1800 - 1914' - chapter 12 'Patents & Techniques'.
In this respect, these volumes from Gulliver and Hajdamach are complimentary - the latter providing much historical text and photos of machines etc., whilst Gulliver provides high quality pix and specifications of the end result.
Fran's comment about the colour of the pontil scar seems to show that this discolouration is probably not uncommon - one of the above pair has a sort of dirt encrusted scar.
Charles Hajdamach's book, for those who collect C19 glass, is another essential volume, and very good value for money.