The body for a sound field microphone (1st order ambisonic A format). 
Designed to fit JLI 120 A-T capsules.

https://www.jlielectronics.com/microphone-capsules/jli-120a-t-43db/

Not yet printed, so still WIP. 

Based on the Gurzon papers discussed at the soundfield website

https://intothesoundfield.music.ox.ac.uk/tetrahedral-recording