Guessing it's a blocking gizmo that prevents the hammer from striking the valve or something along those lines. Might be as simple as a piece of poly pipe or some sort of impact absorbing polymer buffer. Well worth trying a Tau for yourself, or any other pistol for that matter. New is kind of nice because you know it's history, s/h deals can work out well & hopefully it would have been well looked after & serviced by someone who knows what they are about, but sometimes things don't come to light till later.
A 5, a 5, .dodgy pellet! Did you call it?