I used the red FPJ and BH209 with average success. It just required more attention to the plug cleaning. I added a Lehigh version of the non FPJ to a LRH and it fit perfect but some might need minor fitting. I tried mine with 3 different bolts and they all worked including the jeweled version.
I dont think the Knight version states that about the LRH but i could be wrong. The one on Lehighs site did list some DISCs versions might not fit.
From Knights website
* Converts Knight Rifles that use a Full Plastic Jacket to utilize a bare 209 Primer
* Fits: Extreme, Long Range Hunter