Having struggled to get any form of sensible answer from the manufacturer - were do I go to identify the etched numbers on the bottom of several items of Stuart crystal Amber glass? The numbers appear to be etched in using acid, the finish is too poor to be laser and the glass is (According to the family) too old to be anything other than acid. The numbers are all six digit, such as 681649 and 681310 - I woud be interested in any clues as to age and marketability of either individual items or full (6) sets.... many thanks....