I have a Mercury that was tuned (And customised a bit!), It had an axis bolt/nut fitted among other stuff done, It was done by a gent called Ed Bishop, I think from Essex, It's a lovely rifle and much more positive on the lock up that the other Merc's I have
I was told (If my memory serves) that the axis bolt assembly came from a Haenel but it looks more like a HW affair to me!