APM 2.5 latest possible Airplane SW

I reactivated my APM 2.5 or 2.6, I am not sure. Now I want to use the correct Firmware for my plane. I read that Arducopter 3.2.1 is the latest possible Firmware for APM2.x for copter use, but did not find any info about which Arduplane Firmware is the latest possible one for These boards.
Can anyone help?