Let me rephrase my response.
What you refer to as the "rough infill" is the residue left by another piece of glass that was previously attached or inserted by person or persons unknown and quite possibly the glassmaker but equally possibly a previous owner or seller. Whatever the purpose of that secondary piece of glass, which may have been functional or simply decorative, it has been broken off!!
I think it was a stopper, having made quite a few bottles with stoppers over the years. My opinion is also based on the fact that many people have brought their bottles back to me over the years and many galleries and shops have asked me to remove and replace stoppers that they or their clients have broken off both my bottles and other people's and their main issue is how to remove the broken off element without damaging the bottle, or art glass vessel/object! They do look rather similar to the state shown in your image
Sometimes it is easy and sometimes it is quite difficult.
Appearances can be deceptive. :angel: You have it in your hands, have another look at it. Or next time you're up in London, bring it over and I'll have a look at it.
But I realise that I don't know everything and I am quite happy if you or anyone else has a different opinion on this and would be just as happy if I were proved incorrect and it could after all be a piece that has broken off an alien spacecraft or a fragment of a meteorite!!.
Apologies for a little bit of teasing!