Spanish Civil War. There are a few different designs for the town of Ayamonte.
Certainly sold to collectors, also sometimes used as a variation on the local "postal tax" theme.
These are found with and without a Viva Franco overprint. As you'd expect, there are specialist catalogues, but they are generally ignored by the majors. The stamps run into the thousands of varieties across Spain.
For those interested in the topic and these stamps: an excellent work in Spanish about the obligatory labels that levied taxes to sustain charitable organisations such as the cocinas econůmicas (something like a 'soup kitchen' for the poor). This is a document about the Huelva province in southwestern Spain.