Royal Armories just uploaded a video about one of these: https://www.youtube.com/watch?v=6-llVl1ofUU

I say "one of these" because we can pretty conclusively say there are at least two in existence. The first pic here looks identical to the gun in the video but the second pic is definitely a different example, with a number of detail differences (grips, safety, hammer) and more importantly a different serial number. Jonathan doesn't know much more about it than we do (maybe less, considering he doesn't even know what to call it, while the first pic here at least establishes the "BDA-9C" name) but he's confident it's a factory piece and not a custom, and the better-quality look at it in the video makes me agree.