In a nutshell, RS is an appearance package (except '67-'68 F41 as Dwight pointed out) and SS is a mechanical package. Since Z-28 includes, or is primarily, a mechanical package, it "trumps" the SS package.
This is an enduro bumper. $85K will buy this '69 Z-28.
Incidentally, I have some rare literature and documents streaming in on ebay. I recently read the procedure on the 1971 Mopar "B" body version of this bumper. There is 1/4" of one coating followed by 2 1/2" of the elastomeric coating! Talk about killing the weight distribution!
Here is a real '69 RS Z-28. Follow the link for more pictures and a very thorough description. Would love to have this car in my garage!
http://cgi.ebay.com/ebaymotors/ws/eB...t_44453wt_1161