That's because they are all 904s. They are all based off the 904. The 904 simply grew and changed as requirements grew and changed. Just like the A500 is a 904, simpy with an overdrive unit made into it.
I think you made my point. I thought the number change.....500, 999, 998.... indicated a change to the standard 904 and that the change was that it used a lockup converter and/or had an overdrive. Is that not true?