Probably a complete non-starter, but it's an intriguing piece, and unusual in some ways.
The wear suggests some reasonable age - might even be back in the 1930 - 40 period, with what appears to be a fire-polished pontil scar, certainly no sharpness to it. Diameter is about 11.5" (285mm) - and not a very long ring when flicked.
But it's the extent of the tool marks around the bowl that are most noticeable - so am I correct in saying it was blown initially, then detached from the pipe and attached to the pontil rod - spun or tooled into shape whilst hot and still attached to the pontil rod? Think the start and stop shears point can also be seen on the rim. It's quite a heavy piece so not easy to work.
No stones, but some small pieces of frit adhering to the inside, and it goes without saying there aren't any marks apart from that odd looking almost birth mark like spodge.
So, nothing to do with a mould - and seems very unusual with that fire-polished pontil scar left visible.
If anyone has any thoughts would be pleased to hear, and thanks for looking
