According to Edith Reiter's 2014 book, this instrument was completed on 19 March 196 and sold to a customer in the Czech Republic.
Howe & Hurd (2004) describe heckelphone #18 as a model 36b. As of 2020, it is part of the musical instrument collection of Universität Tübingen, Germany, where is has been assigned item no. MWI-Is-A100.
Previously, heckelphone #18 was part of a large collection of historical wind instruments assembled by Dr. h.c. Karl Ventzke and donated to the University of Tübingen in 1999. The earlier history of this instrument remains unclear.
An official picture of heckelphone #18 can be found here.