Scott,

  Not the whole story;

Here you're referring to 'salt-flat' or 'racing' disks (some say
'Full-moons') when you speak about the spun aluminum, almost flat,
featureless disks that cover the entire wheel.
ttp://cgi.ebay.com/Model-Car-Parts--Salt-Flat-Moon-Discs_W0QQitemZ350021215890QQcmdZViewItem?IMSfp=TL0802020976a11235

'Baby Moons' is the common term used (and abused!) in many automotive
circles (not originally VW's) to refer to  the dome-shaped caps used in many
older cars, including older VW's.
 http://www.hubcapmike.com/baby-moons.html
or like this
http://cgi.ebay.com/ebaymotors/NEW-BABY-MOON-HUBCAPS-MOPAR-CHRYSLER-PLYMOUTH-DODGE-8_W0QQcmdZViewItemQQcategoryZ33744QQihZ022QQitemZ350020379768QQrdZ1QQsspagenameZWDVW
 or this
http://cgi.ebay.com/ebaymotors/10-3-16-CHROME-STEEL-BABY-MOON-HUBCAPS-WHEEL-COVERS_W0QQcmdZViewItemQQcategoryZ43961QQihZ018QQitemZ280196985971QQrdZ1QQsspagenameZWDVW

Mike B.

Just for your automotive and cultural historical  correctness, 'moon
hubcaps' cover the entire wheel.
I believe the origin is hot rod cars that ran on dry lake beds in southern
California, after WWII and at the start of the hot rod era in the late 40's
and into the 50's...........on those cars for streamlining purposes they
covered the wheels over smoothly with probably aluminum full size hubcaps.
And possibly first used on 'dry lake' cars based on, or build upon, aluminum
surplus aircraft belly tanks.