That black coupe was me 🙂 the brake light wiring has been stuffed into the spoiler and isn't connected at all. The spoiler has been silicone on or something as well.
I originally thought it was oem on the car, given what i understand about the cars history, until i discovered the wiring and the silicone holding it on. also when you open the boot and look under you can see that the moulding on the spoiler is squared off, where the boot shape on the c70 is rounded.
i tried to illustrate the contour of the c70s boot in the below photo in red, and the contour of the spoiler that visible and doesn't line-up in blue.

Bolts on fine, and i havent had the opporunty to investigate the setup on a c70 convertible, so i have no clue if the spoiler actually ever came oem on a c70 coupe. Its entirely possible that it came on the c70 coupe, just the contours never really lined up well.
Definitely fits on the car, just an arguement as to if the coupe ever came with them to factory, I'd love to know if the coupe did.