Just a Thought that I just came up with after reading all the comments about the Break indicator light on my 150 mph speedo that is lacking an odometer !!
If this were an aftermarket speedo wouldnt it be lacking the brake light indicator or shouldnt it at least be differant then an original Camaro speedo ? http://hometown.aol.com/natdevris/myhomepage/auto.html
If it is an original speedo it should have a part number and a date code on the back of the speedo face. If you do pull it then you will be able to tell if it has an odometer in the speedo head or not. You might get some more info from the tag that was placed on the cluster housing.