The knurled nut goes behind the muzzle break, you can see a small space for it in your pic. I would have to look at my refs for the PE as I cant remember off the top of my head. Check here and click picture to maybe get some hints
Thanks James! That's the right set & it looks like I'm missin' a couple parts.
![Blink :blink :blink](https://modelersalliance.org/images/smilies/blink.png)
![Bangs Head :bang head :bang head](/images/smilies/dash3.gif)
Please check your references when you have time James, I'd really appreciate it. Back when I built tanks, we didn't have all these goodies!!