Thanks for the reply.
I'm asking because I just set up my own nitro tap. I took it apart and there were both Orings and the plastic bit beneath the restrictor. When I pour at 30psi, I get nothing but foam from the tap. I suspect that the stout is over-carbed. It was carbonated to about 2.0 volumes with pure CO2, then I purged and put it on the 70/30 beergas. I'm going to try and decarbonate it down to about 1.6-1.8 volumes, then give it another go.
Do you know what size of holes the new plate has?