(read how to identify your stamp below)
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 here.
The stamp did have a secret mark, because it was not always used, I have not referred to it.
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.