I have tentatively dated these tongs at 1767.  Richard Mills entered his first mark in 1755, and several others thereafter, the latest being 1767.  Grimwade records some confusion over this maker & his dates, however, the mark is very clearly his.  You can also see a pair of tongs by Thomas Hannam & Richard Mills, a mark that has some doubt over it.  Frankly, from the overall style of the cast tongs, I would suggest they be dated around 1770 - 1775.  So was Richard Mills having work assayed this late?

 

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1767 

1767 

1767