The difference is the gas cylinders slightly modified with a larger gas hole to adjust to the 16 inch barrel and it uses a one piece gas lock and muzzle brake. Everything else is the same.
If he's selling the barrel, muzzle brake/gl and cylinder than it's simply having a smith put on the barrel to your receiver and headspace the/a bolt to it, use that gas cylinder with it and muzzle brake/gas lock combo and everything else is the same.