British Design Registration Numbers
When you see the RD diamond symbol, it will tell you that the design was registered between 1842 and 1883. The protection could last for up to 25 years, so it is possible for a design symbol to continue to appear, although changing styles would likely preclude continued use.
Beginning 1885 a numbering system was used
If you would like to research your design number, follow the link to the UK Patent Office. They offer a search option on their web site.