(read how to identify your stamp below)
You have the 12c Clay from the years 1870-1873. It is uncommon. To value the stamp you have to first look for a what is called 'A Secret Mark' and look for an embossed pattern, called a grill, on the reverse of the stamp. An example is shown below.
If the stamp has a grill, then click here.
If it does not have a grill and balls at the end of the number 2 (shown below) then click here.
If it does not have a grill and crescents at the end of the number 2 (an example of this secret mark is shown below) then click here.
This stamp was reprinted in 1875 and it can be viewed by clicking on the numbers 167-177. These are all accounted for, so you will not own this.