Ok, Peter, thanks. I bow to your vast and superior knowledge of really old glass and drinking related themes.
I didn't think that seals impressed onto blobs of hot glass on a hot bottle would come off, sort of intact.
I assumed they'd end up breaking bits of the bottle off with them, or leaving bits behind.
I have no experience of how glass behaves over periods of hundreds of years. I know you do, (but it has nothing to do with your personal age).