According to SOS's description, "ScienceofSpeed set out to make a direct reproduction. The factory NSX-R cover was replicated down the smallest detail. The cover, with its internal strengthening frame was reproduced using a factory NSX-R cover. This allows the cover to be lightweight, yet rigid. The frame is made with a carbon fiber base, and is available with a black satin painted FRP or exposed carbon fiber shell."
And looking at Procar's description of selling hinges/spacer. (see pg 57), I think the SOS NSX-R cover should fit a targa nsx. However, the fitment may not be perfect or up to SOS's standard which is why SOS is not promoting the cover for the targa. (my guess) Lots of NSX owners are picky to the micron so if there is a slight miss fitment, panties get bundled and SOS doesn't want owners to blame them for preaching something they can't back up. Unless SOS's NSX-R mesh cover is a different design than OEM spec, then my assumption is squashed.
I have a Gruppe-M NSX-R style cover which is a similar replica to the OEM NSX-R cover except for the prop rod gap (does not exist on GM). Without spacers, fitment would be impossible due to the raised bulkhead as mentioned. By using spacers, this raises the cover enough for a better fitment, but not perfect. There is still contact between the under skeleton strengthening support of the cover and the bulkhead, but not enough to cause damage or to prevent closure of the cover & hatch.. Unfortunately, the cover has a slight flex of a few millimeters and you can see a slight "bowing" if you look close enough w/your nose physically on the quarter. (that's being picky)