If you have an old piece of metalware, ceramic. plastic etc with a 'Rd No' marking it is almost certainly of British origin and the numbers that follow it can accurately help you to date it. They were in use from 1884 to 1949 on all kinds of things from dolls to doorknobs. The following list gives the range of numbers followed by the years that they were registered:

1-19753: 1884
19754-40479: 1885
40480-64519: 1886
64520-90482: 1887
90483-116647: 1888
116648-141272: 1889
141272-163766: 1890
163767-185712: 1891
185713-205239: 1892
205240-224719: 1893
224720-246974: 1894
246975-268391: 1895
269392-291240: 1896
291241-311657: 1897
311658-331706: 1898
331707-351201: 1899
351202-550999: 1900-1909
552000-673749: 1910-1919
673750-751159: 1920-1929
751160-837519: 1930-1939
837520-860853: 1940-1949