2006 Forester- water pump part #'s

I'm having a full headgasket job with new timing kit done on my 2006 Forester, EJ253 N/A engine and automatic transmission. I had noticed the Aisin TKF-006 timing kit that fits most 2006- 2010 EJ253 engines specifically excludes the 2006 Forester with automatic transmission. I investigated further to find that Subaru lists OEM water pump # 21111AA290 for my car where all other 2006- 2011 EJ25 engines use 21111AA280 (which is apparently the water pump that Aisin stocks in the TKF-006 timing kit).

Does anyone know the difference in these water pumps and why the 2006 Forester with automatic specifically would use a different water pump (which appears to be a one year only part)? I decided to play it safe and ended up ordering an OEM # 21111AA290 water pump, but the part now seems to be in an infinite delay thanks to USPS. It would be great if my mechanic were to be able to install the 21111AA280 part that comes with the Aisin timing kit, but I don't want to risk any further problems with this car (it already is overheating due to internal headgasket leaks from both heads per the pressure test my mechanic performed).