There is a difference, it has been up in a topic a long time ago, with nice pictures. It�s no difference in sound (if I remeber right) only in the layout of the PCB. But I can�t find it with the search engine...
The layout changed a little around serial 9000 when the permanent LED was introduced. Apart from that I think the brand of capacitors has changed several types so when you open them up and compare they will be different colours.