You have the 2¢ Jackson from the years 1870-1876. It is quite common.

If the color of your stamp is closest to the brown of the stamp on the right, click here.

If the color of your stamp is closest to the red-brown of the stamp on the left you should look for an embossed pattern on the back of the stamp, called a grill, an example is shown below.

Example of a grill

If the left stamp has a grill embossed on the stamp (shown below), click here.

If the left stamp has NO grill embossed on the stamp click here.

This stamp was reprinted in 1875 in a dark brown shade (shown below) and it can be viewed by clicking here. These are all accounted for, and will be accompanied by a certificate stating it as being #168.