Think that they have only recently surfaced through Sure Shot airguns, and I have never seen one except in pictures.
I was talking to the sure shot guy at the Harrogate show and he was pushing them for FT/HFT. The mamba action that they are bringing out looks slightly better with the side lever set up to me, but again I have not seen one. Their Ultra AA 400 action is a nice piece of kit though.
Too new to tell I think, although with the name on it it should be something like.