There are two things that feature prominently in the Mark Degree, firstly the keystone, secondly a Mason's Mark.
In a similar fashion I have used my own Mark on all of my websites, that's that odd looking thing at the bottom of the menu, over there on the left. It might be that someone, somewhere in the world has a similar, or may be even identical Mark, but within my own Mother Lodge of Mark Masters it is unique, and I could if the need arose be identified by its use.
The story is based on the preparations for the building of King Solomon’s Temple and follows the fate of an ambitious Fellow of the Craft (the Candidate) seeking promotion in his trade by demonstrating his skill and ability by fashioning a particular stone (yup, you guessed it, the keystone that you keep hearing about). In the early part of the ceremony his talents go unrecognised and his hopes are dashed but eventually he triumphs over adversity and is justly rewarded for his work and made a Mark Master.
Apart from obviously being a special stone, what exactly is a keystone..?
The keystone is the centre stone of an arch. See the diagram, which illustrates quite nicely the importance of the keystone in the structure.
The Mark Degree is believed to have originated as a ceremony of registering a Craftsman's Mark in those years distinguished by Operative Craft Masons and their Temple building. It was later developed into the full-fledged degree as we know it today by the Masonic fraternity . Some scholars say it was the earliest degree and may predate all others by many years