If it was ever designed to fire powder cartridges, it would have a removeable grommet or similar to allow the air to escape in front of the piston when fired in 'firearm' mode - looking at the pics there seems to be a hole at the top of the cylinder, covered by the T-bar. Could this be it? There also seems to be an indent next to the breech seal, which I'm guessing might have been to help with removal of the firing pin attachment?

In any case, in the absence of a firing pin and barrel liner, it is surely an airgun and not a firearm that would need to be on an FAC, especially if it was sold as such. That's what I would argue in court, anyway.