I had looked at that Frisby soda while active and was drooling! Gorgeous soda! The pontil on mine will cut your finger-not the average center of an improved pontil. Kinda looks like they tried the "new-fangled improved" pontil and either didn't hold or decided to switch back to the tried & true method. Whatever the reason, I couldn't make a call which one it looks more like! Cuts your finger in the center and has residue surrounding it. To my eye it looks like the protruding glass was added over the iron pontil.....Jack
I sold a black glass whisky cylinder about the same time as the Frisby that had a mix open / iron thing going on. The outside looked etched like an iron, while the middle has a huge slag of glass. The chunk of glass wasn't sharp, very funky looking pontil though.
Oldest looking bottle I ever dug was a black glass ale likely from the 30's had what you describe. So crude and out of shape! Very crude gnarly looking pontil.....