At first, these marks were often a symbol of either the maker's shop sign or a rebus (pun) of the maker's name, but gradually during the 17^thC initials, usually embellished with symbols, became more common.
The Huguenot craftsmen who registered their marks tended to follow the custom of their native France and use a fleur-de-lys and two pellets (or grains) above their initials.

In 1697 when the Britannia standard came in to force, makers were required to re-register their marks in the form of the first two letters of the surname.  With the re-introduction of the Sterling Standard in 1720 makers were required to mark their Sterling wares with the initials of the forename and surname but  makers continuing to use the Britannia standard used the first two letters of the surname.

The Act of 1739 ordered that workers should destroy their existing marks and substitute others with the initials of their Christian names and surnames in letters of a different character from those used previously.  This form of mark has been used until the present day