My guess is that any "plastic' sole material was installed by a prior owner. EY would not have done that and I doubt that the material was in production when the boat was built in the late 70's.
A decision to restore the sole to a 'real' teak and holly surface or to a modern 'very real looking' teak n holly plastic will need to be made after you do get access to your bilges.
Note that whatever you put down for a new sole needs to be secured with fasteners that can be easily removed.