It's a lefty, walnut MarkI that I've been neglecting. I bought the gun, with at least a couple hundred thousand shots on it, from an old FT shooter. It had a plastic tophat that had become stuck, so I put it aside. I finally got the top hat out of the MarkII FAC piston that came with the gun. These are about 93 mm stroke, so just 5 mm shorter than the MarkIII. I changed this for a standard 84 mm stroke MarkI piston and used the seal that was on that piston. The seal was a little less tight and of a harder material, but it sealed perfectly. The gun was quick with a sharp shot cycle that I just didn't like. I pulled it apart and put the seal from the first piston, which is more pliable and tighter. The shot cycle is soft and smooth, but a little more lazy. That's the way I like them. I'm always amazed that just a small change can make such a difference to the feel.