The flat "beaver tail" is newer. It is aerodynamicly better. MDC put it first on the MD 87 and later on the newer versions of the MD 81-83. You can see the same difference between the DC 10 and MD 11. BTW the APU outlet of the MD 80s is not there, but higher up.
What you say dosen't add up because, the flat "beaver" tail is in an older pic than the round cone shaped tail. So, it would seem the cone tail has replaced the flat tail. Follow the links for other pics of the same aircraft. Use the reg# as your search topic.
Nevermind political correctness - Envision using your turn signals!
The beaver tail is newer than the cone. Try looking at pictures of the 717 versus the DC9 and you'll see. On N289AA, maybe the beaver tail was damaged somehow, and it was replaced with a cone that was already in inventory.
The cone shaped tail is definitly older, these are the older aircrafts. I know it because Swissair had first aircrafts with cone shaped tails and later newer with beaver tails. The first beaver tail was on the first MD 87. About the APU you are right I told it wrong.
The outlet you see below and forward of the N number is acutall an exhaust for the heat exchangers/packs. The apu exhaust is on the right side of the fuselage just forward of the right side pack exhaust.