I've heard those aftermarket tops called "Sebring" tops, rather than "Surrey".
Well, actually Triumph themselves never called the factory version of the two pieces shown in that auction (backlight and roof panel) anything more than a "hard top".
The "Surrey" was actually a separate "weather" accessory for the hard top: a folding vinyl roof panel that could be stowed in the boot in case a rain storm came up while driving around without the rigid roof panel.
The backlight of the "Sebring" is lighter in weight than the originals, mostly because of the plexiglass window. That and the lower cost made them somewhat desirable for racers.