Android N Deveoper Firmware for Shield TV 2015

I see the developer firmware running Android N (7) with nVidia experience 5.x.x is available for download for the 2017 Shield TV (the new hardware) but I don’t see any firmware for the 2015 Shield TV. Does anyone know if the 2015 box can use the 2017 firmware (I’m assuming not) or if there is developer firmware for Android 7 (experience 5.x.x) available somewhere for the 2015 device?