They look like 66 Chevelle SS hubcaps to me. I've never seen these on any Buick................http://chicago.craigslist.org/wcl/pts/2862378960.html
I think someone had posted a link to an ebay sale for a similar set, and I recall someone saying they're pretty rare. but they only sold for like $50. I'm almost certain a board member bought that last pair.
I think this is the older thread; different design: http://www.v8buick.com/showthread.php?242609 Devon
These were GM Dealer Optional Equipment/Accessories and several Divisions had these wheel covers (called "Simulated Mag Wheel" covers) with the appropriate division emblem in the center. These were by far more common on Chevys, but I have seen quite a few on Buicks including a '69 Skylark Custom conv I purchased years ago from the original Owner. He had the accessory brochure with these wheel covers and a host of other Dealer add-ons (from Buick) pictured and described. That's how he bought the wheel covers ... I ended up changing over to chrome Road Wheels anyway. As I recall, these were really from the mid 60's to late 60's offerings.
Another set was on Ebay a while back: http://www.v8buick.com/showthread.php?109132-How-many-of-these-have-you-ever-seen&highlight=
Here are some pictures I found ... you will recognize these wheel covers from the 1965 Chevelle SS 396 Z-16's as well as later year Chevelles, Impala SS's and othe GM Division vehicles. The center medallion 'pops out' - as I recall - and can be replaced with another Division logo.
