The "pattern" on the underside looks to me like the normal semi-random pattern you get when laying chopped-strand matting into wet resin and daubing it with more wet resin, the old-fashioned way, ie by hand.
And now somebody has mentioned a headliner, perhaps that is how the original makers (probably a small subcontractor to Abingdon) covered up the ugly rough inside face of the shell.
I'll report back when I have the glassfibre guy's views and an estimate from him.
I had half thought of making a replica of it using it as a mould, and a lexan window. Today's resins are so much better, it could be made much lighter. Ideally carbon fibre would be cool but pricy.