According to Edith Reiter's 2014 book, this instrument was completed on 4 July 1906 and sold to a customer in Germany.
Howe & Hurd (2004) describe heckelphone #18 as a model 36a. As of 2020, it is part of the collection of historical instruments of the Museum Viadrina> in Frankfurt/Oder, Germany, where is has been assigned item no. V/J 177. The instrument has been acquired 1975 by the museum as part of the private collection of Berol Kaiser-Reka, who may have purchased it 1974 from an unknown source.
According to information provided by the museum, heckelphone #21 was used until about 1945 by the varieté arists "Die Soundy's" in Dresden, who purchased it directly from Heckel.
An official picture of heckelphone #21 can be found here.