You have the 15c Webster from the years 1870-1879. It is fairly uncommon.

If the stamp has an embossed grill (an example is shown below), then click on the numbers 134-144 on the menu to the left.

If the stamp does not have a grill embossed on it (an example of a grill is shown below) then click on numbers 145-155 on the menu to the left or if the paper seems that it is a soft porous paper and does not have a grill, click on the numbers 156-166 on the menu to the left.

The stamp did have a secret mark, because it was not always used, I have not referred to it.

Example of a grill

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.