The subcontrabass flute may or may not be exactly the same thing as the double contrabase flute. That depends on who you talk to about it, and how knowledgable they are, and how prone to suddenly become violent and attempt to pummel you for raising what they may regard as the spectre of heresy.
Paige Dashner Long [pictured here], Director of the Metropolitan Flute Orchestra in Summer Residence at New England Conservatory: “This unique instrument is very versatile, and fills the gap between the Contrabass and the double contrabass. In choirs it supports the Contrabass, and it literally propels the double contrabass.”
This video shows Stefan Keller playing, to the best of his and the instrument’s ability, a subcontrabass flute: