I didn't see the pic that you're referring to in this post, but my all-original 1 owner (b4 me!) '87 Westy GL has one.  It's the same color/ material as the rest of the curtains and attaches with velcro to the skylight-surround.
  My velcro strips on the surround have peeled off due to age, but I've bought some more with double-sided tape at Mal-Wart to replace it, so I can actually use it.  It's not only for prying eyes (which would be tough), but for blocking light.
  IIRC, there's also a bug screen for it.

Mike B.