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30 May 1775--Joint mark as bucklemakers of John Jones & John Chadwick at 46, Minorities
.5 May 1777-- Sole Bucklemaker Mark of John Jones at 46, Little Minories
30 May 1777 --Sole mark as bucklemaker of John Chadwick at 30, Minories

Presumably the partnership had been dissolved and John Chadwick moved down the road !
He did not stay there long .
16 July 1779 --Chadwick informed Goldsmiths' Hall that he had moved to 6, Raven Row, Spitalfields
He then moved again as on 18 February 1784 an advert appeared in Morning Post and Daily Advertiser
.... "PATENT KNEE BUCKLES, So constructed as to not catch the Stockings or Cloaths.. .. ..
.. ....CHADWICK's , The corner of BIRCHIN LANE, CORNHILL"

He then appears to have taken a partner, Joseph Hillier and gone bankrupt !
21 June 1788 --The London Gazette gives notice of bankrupcy of
"John Chadwick, and Joseph Hiller of Birchin Lane in the City of London , Jewellers, Dealers, Chapmen and Partners"
a later announcement on 27 January 1789 in the Gazette gives them as Hardwaremen, Jewellers, Dealers, Chapmen and Partners"

Grimwade did not provide an image of the mark of the partnership of John Jones & John Chadwick.
However he gave it, page 394 , as a square-cornered mark with a curved top, registered 30 May 1775 .
The photo below is almost certainly this mark, seen on a pair of cast tongs .
I have never encountered this mark on buckles, and am very jealous of my wife who recently bought the tongs !
Two views of the same mark.
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Recorded as a Goldsmith and Jeweller of 36, Cornhill.

Source: Johnstone's London Commercial Guide - Corrected to 31st August 1817 - Andrew Johnstone - 1818