Hi
These stamp is effectively a revenue stamp, but it has a postal postmark.
Was printed in 1878 by American Bank Note Co.

In those years when some face value (in postage stamps) was sold out, it was authorized the use of revenue stamps for postal use.
In the stamp shown before the postmark indicates 18-FEB-1881
Even in your postmark the date is inverted in relation to circular frame (Iquique-Chile). Nice error !
This are the dates of use for the shown stamp:
11-27-1880 to 8-8-1881
4-21-1891 to 9-30-1891
1-20-1892 to 9-14-1892
10-25-1900 to 3-31-1901
Revenue stamps of 1900 are different. They have another design (face value in 4 corners) and were printed by Waterlow & Sons.
Like the first, were use in postal service too (1900-1901 and 1913 to 1915)
