I have several A600s. The only differences I've seen is in the IDE connector. Those that shipped with a HD (Model A600HD) had all the pins, including the pin for the HD activity LED. I have an A600 that was missing that same pin. I have no idea why this was done. After installing a HD, I also soldered in the missing pin and everything is fine.
Some of the keyboards "feel" better than others. I like the ones with the clicky tactile feedback.
