A Russian Royal presentation brooch pendant, early 19th century, the circular brooch pendant with

A Russian Royal presentation brooch pendant, early 19th century, the circular brooch pendant with diamond-set 'A' beneath a crown, referring to Tsar Alexander I of Russia (1777-1825) on a ground of blue enamel, with gold foliate border, the outer edge set cushion-shaped diamonds, mounted in silver on gold, to a hinged enamelled gold pendant bail, inscription to reverse 'This Locket and a pair of Earrings were made from a box presented by The Emperor Alexander of Russia in 1812 to R. Adm'l Thomas Byam Martin for the defence of Riga, and other services in the Baltic', length 6.5cm including bail, fitted case by Sheppard, Goldsmith & Jeweller to the Queen, 63 Brook Street, Hanover Square, London