A note in my lists of makers records an attribution of AG in a rectangle with a star on a 1785 spoon to the Edinburgh maker Alexander Gardiner or Gairdner. Unfortunately there's no cross reference to where that information came from to point you to a reference book. The mark doesn't appear on the Edinburgh web site of makers so at best the attribution is a possible.
Graham , I suspect that the mark to which you refer is your collection tongs number AGX85200407 . It's virtually sure to the same makers mark as this spoon and as it has a duty mark and a thistle confirms , this mark is almost certainly Scots. Certainly the tongs are! But you attribute the ELG also on the same tongs to a retailer's mark. I would be a bit optimistic and suggest this could be an Elgin smith assaying in Edinburgh.
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