Paul & Jonathan - thank you, this seems to be the most likely explanation.
You raise an important point concerning nomenclature, Jonathan - the word ‘acoustic’ does carry a certain value here.
Concerning why an adjective might be added to the name of a stop, the funny thing is that we find qualitative adjectives completely normal for Harmonic, Doppel, or Lieblich Flutes, Open, Stopped, or Stentor Diapasons, etc, let alone adjectives concerning their pitch and genre.
So, if a Trumpet and Harmonic Trumpet, why not a Bombarde and Bombarde Acoustique?
Perhaps it is because - sometimes - such half length stops are seen as compromises? I rarely hear someone arguing positively for a half length stop!