"Center caps" as posted. Hah !! I now have 3 sets of STS center caps - all different !!! Black lettering on chrome, black on matte chrome, black on matte gray, all the "OEM" clear plastic faced ones. Picked up 2 sets at swap meets for $2.50 a cap.
Here's an old picture of two of the STS types. There are others ............. (the CTS caps are the ones on the car now, black)
........... and, no: I'm not going to put Baby Moons on it, and if you think the 17" wheel option for STS had the larger "hubcaps" like earlier 16" wheels, then you're seriously mistaken. The 2003 CTS 17s are the same wheel, except polished/clearcoat - not chrome.