Check this out:
http://parts.nalleygmc.com/showAssem...ey_trimLevel=0
I punched in 2003 to try to find the diagram of the old-style mirror, but only the newer one shows up. Anywho...item #1 is the mirror, #2 and #3 (which appears to be the 'round attachment piece you mentioned) are not listed, but the description for #1 indicates that the 'HTR PAD and GLASS HOLDER' are included. I'm thinking #3 is the 'glass holder'...