The Isthmian script (a.k.a. La Mojarra script and the Epi-Olmec script) is a Mesoamerican writing system which was in use in the Tehuantepec area (of current-day Mexico). The time of its use is not known - perhaps between 500 BCE and 500 CE.

It was first discovered in 1907. Unlike the similar Maya Scripts - which have in many cases been deciphered - the Isthmian script remains obscure - despite (disputed) claims from several investigative scholars.

Further info : Wikipedia

Extensive Technical details : Yet Another “Decipherment” of the Isthmian Writing System , University of Bonn, Germany (12MB .pdf)