Yes, Tom is right, that is the later, tenite seat, I have worked on a couple of glass Heddon Pals that had it. I never had to do more than clean the seats, though.
Sinclair in his handbook bible, states that, in 1939, when they changed the markings on their bamboo rods, they also introduced a new reel seat made of tenite. This included the barrel and down locking threads as one piece, and blackened nickel silver hardware. The marbellized brown color was the most common, but they also made a coral-colorized version for their number 14 rods. Sinclair also states that some of the seats were made of black pyralin to be used with their number 17 "black beauty" bamboo rods. I am not sure if these pyralin, or coral tenite seats were ever adapted for use on their fiberglass rods, but the brown ones were definitely used on fiberglass Pals in the 1950s.
In any event, from a refurbishing standpoint, Sinclair lumped pyralin, and other plastics into one category which he said were the hardest to work with. He stated that removal only worked half the time for him. He recommended using the seat in the boiling pot of water trick, but he was reticent about it.
He also noted that the heating process may cause a lightening of the material. He found that the fix was to use simichrome metal polish on the tenite, and a lot of "elbow grease."
Personally, I would suggest a dremel, but I think that CroixBoy has it right, do the cut and stent thing. John