The Hern NSSA style breech plug is thicker than their standard plug, and is dished rather than flat to allow it to be cleaned easier. Just tell Joel that you want an NSSA style breech plug and he will understand what that means.
Before you order though, ask the other vendors here what they can do for you and see what you like best. Something as simple as your location, which determines shipping charges, could make a big difference in price from two different vendors.
And certainly if you can get something machined from solid steel rather than cast around a steel liner you will have a much better gun.
Lots of variables... time to do your homework!