The photograph below is an example:

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Here's the answer....from 1842 - 1883 the British Patent Office decided it would be a good idea to issue a diamond mark as well as a registration number when a design was registered. This is helpful on a number of scores. For one thing, it confirms that the design is British.   And with the number of fakes flying around, this can only be a good thing. Then there's the added benefit that an item with this mark is unlikely to be a copy which is very reassuring.
And of course, the presence of the diamond gives the instant information that this is an old piece, as the window of time that the diamond mark was used is relatively small.

Further examination of the mark will yield much more than that, though.

• First of all there are two ranges of year codes. Sometimes you will see a number in the right hand angle of the diamond and sometimes a letter. The difference is that the numbers signify a range between 1842 - 1867 whilst the letters were used between  1868 and 1883. The illustration is here is of an item registered between 1868 and 1883
  

• The Roman numeral at the top represents the class of material eg glass which would be III, or clay as in this case, is IV.

• The number in the left hand angle denotes the number of these items were in the registration process at that time, also known as the Bundle.

• The Arabic number at the top is the day,  the bottom letter represents the month and the year is shown by the letter in the right hand angle.                                      

• The mark in the photograph shows that this item was registered on the 28th November 1879.