There is more to that conversion than 'a new gun barrel'. The J turret was modified on its front face and roof, and side doors and pistol ports were added as well as provision for fitting the early-style . New mid-series road wheels are included along with a new drive wheel. On the rear I've added the smoke candle rack and turret traverse muffler, while I include a mounting step for the left side. There is, as you noted, a new gun barrel as well as the unique ball-shaped muzzle brake.
Maybe you should have taken a closer look while you were there; that's why I brought the example along to the show. Anyways, since you missed some things, here's a photo.