It's awfully nice of you to spend so much time thinking about my question. I should have mentioned earlier the meaning of 'R88' was clear. Incidentally, it is not explicitly prescribed by SARPs but by Attachment A to the EUR ANP, Volume II, FASID, Part III - AOP. Unfortunately, said Attachment does not help with the 'D' either.
I was beginning to think it might be a typo (perhaps 'R88/CLRD' was meant), but a search for historical METARs has disproved this assumption. Besides, my idea the 'D' might be a German thing is apparently not true since I could find a METAR from Zurich containing the very same 'R88/D'.