The patent number on an object has the letters RD, Reg No or Reg Des. What does this mean?
The letters RD, Reg No or Reg Des (or similar) stand for registered design. These are not patents (which protect the features and processes that make work); registered designs protect the physical look or visual appeal of an object. Tymel Patents (and the Intellectual Property Office) is unable to help with pre 1990 registered designs. You can get an idea of the date from Great Glass; this site lists registered design numbers for glassware but will give you an idea of the date for non-glassware designs. For full information on a registered design you will need to go to the National Archives. They are able to supply copies of the representation (line drawings or photographs) and more usefully the register entry (giving details of the designer and dates, maybe licences) of a registered design. They publish a good research guide on registered designs.