What's the plastic material on the side decks?

[eze2bme]

When I had my back deck enclosed (1979 43DC) with a hard enclosure (glass and mahogany painted with Imron) I used extruded 1/4" PVC to replace the weatherboards - as Walt said - wanted it a little more weatherproof from underside of the handrails to the deck - that was over 10 years ago and still holding up well - had the PVC painted with Imron also.

[JPBuffalo]

Any idea on the screw size that hold the panels onto the poles? Looking for some of the screws.