Looks like the last generation Riviera had this wheel as an extra cost option. Found a pic on the following website:
OEM 1999 Buick Riviera Rims - Used Factory Wheels from OriginalWheels.com
Didn't some newer Rivieras have a wheel like that without the fake rivets the Reatta wheel had? I thought they were chrome. Those on eBay look like they may have been sandblasted. Maybe they were originally chrome.