Where did you replace them from? I'm avoiding random Chinese ebay sellers as I can't tell the quality (I expect an aftermarket 'chrome' back to be poor looking!)
I found this, looks quite accurate to the original back cover... I don't think you can get original back covers without salvaging it from other iPods.
I'm getting this soon Ive seen a tutorial where you can use fine sandpaper to give it a brushed aluminium finish which imo looks really original.