This is ford we are talking about though I've seen a few standard RS1800's with rubber spoilers and mine also had a rubber spoiler and what looked like original plastic covered rivets holding it on. Interestingly though, mine was a 1993 (M) Red car, and many of the cars I have seen with the black spoilers are later cars, and mostly red. My guess is Ford started to run out of colour coded spoilers and still had stock of the rubber ones.

We need an expert on the case lol.