British Registry Mark
In 1842 the British Government established the British Registry Mark
The purpose of the mark was to show that the design or shape had been registered at the Patent Office in London and was thereby protected from piracy by other manufacturers for a period of three years. This appears as a diamond shaped mark and gives the EARLIEST possible date of manufacturer and appears on Victorian ceramics from 1842 – 1883. After 1883 the Registry Number (Rd. No.) is used.
The British Registry Number is the equivalent to our Patent Number and applies to Designs, Trade Marks, Copyrights and Patents. A complete set of printed specifications of patents from 1617 is available in the UK Patent Office Library and since 1854 the office has published a weekly Journal of Patents. The Patent Office also publishes a weekly Trade Marks Journal in which all newly accepted marks are reproduced.
The diamond can be found either printed or impressed. As a general rule the printed marks related to the decoration on the piece, whereas the impressed marks generally related to the shape of the object.
Table 1 was in use between the years
1842 and 1867. 
Year letter at top
A = 1845 F = 1847 K = 1857 P = 1851 U = 1848
B = 1858 G = 1863 L = 1856 Q = 1866 V = 1850
C = 1844 H = 1843 M = 1859 R = 1861 X = 1842
D = 1852 I = 1846 N = 1864 S = 1849 Y = 1853
E = 1855 J = 1854 O = 1862 T = 1867 Z = 1860
Table 2 was in use between the years 1868 and 1883.
Year letter at Right
A = 1871 H = 1869 P = 1877 X = 1868
C = 1870 I = 1872 S = 1875 Y = 1879
D = 1878 J = 1880 U =
1874 
E = 1881 K = 1883 V = 1876
F = 1873 L = 1882 W = 1878
Months (both Periods)
A = December E = May K = November (and 1857) December 1860)
B = October G = February M = June
C or O = January H = April R = August (and September 1st-19th 1857)
D = September I = July W = March