Just bought the RT-AC1900P at Best Buy. It came with 3.0.0.4.380.1842, so I flashed 3.0.0.4.380.3264 and then RT-AC68U_380.60_beta2. I'm still at work, so it's not currently operating as a router or anything, but the flashes went through just fine. I'll hopefully be able to finish configuring it...