Apologies for raising the spectre of this old post, but as will be noted from the Edinburgh index at:
http://www.925-1000.com/dlEdinburgh.html" onclick="window.open(this.href);return false; this mark has now been attributed to Mackay, Cunningham & Co.
James Mackay, a former apprentice of Alexander Gardner, formed his partnership with David Cunningham in c.1824 opperating from 40, South Bridge, Edinburgh. They became Mackay, Cunningham & Co. in the early 1850's and were based at 54, Princes Street. They were taken over by James Hardy & Co.Ltd. c.1912.

Mackay, Cunningham & Co. - Edinburgh - 1859

Mackay, Cunningham & Co. - Edinburgh - 1871
Trev.