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"Another possibility is that it could be from a Roman land site, which is now under water because of coastal erosion...
Have none of those Brit "Historians" ever heard of "Beach Glass"?

That blue glass vessel is in pristine condition. Had it experienced anything close to a millenium of shoreline erosion and wave action, it would have looked more like this:

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Nearly always right -- but always opinionated... '-)
TXnMA

Jellybeans!
Excellent point! :)