Grouse is talking about a red plastic jacket plug BUT the vent liner is not braised in like the OEM Knight plug. You can replace just the vent liner when it wears out. Any primer seems to fit fine in the jacket but in mine mags were much dirtier.

Bare primer and a win209 primer is way cleaner. My Bestill bare primer plug can use either the Fed209A or Win209 perfectly. My Lehigh bare primer plugs work best with a Win209 but they can be shimmed to work with others too.

Both plugs are made of 17-4 stainless and made by Lehigh so either one is excellent quality. Jeff had 100 or so plugs made a little long so they could be custom fit. You will need either a bare primer bolt or adapter too with the Bestill plug. I prefer the adapter.