"IT" is a very difficult maker's mark to attribute definitely. One very likely contender is John Tweedie. This is another of the Johns that is difficult to identify. The mark looks very like John Tweedie’s mark. The doubt arises because John Tweedie only entered one mark, on 1st December 1783, as a plate-worker.
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c1780 - 1784
c1786 - 1790