I'll provide a little inside peek. There are typically 3 classes of builds internally:

-RX34
-RX44
-RX34+RX44

There is some file size difference, but I can't provide more detail than that. However, I can say that on some interim builds I easily managed to install an RX44 image onto an RX34 device with zero apparent problems. I have not tested the other way. RX34+RX44 builds install well onto either hardware. Dunno if that sparse revelation is helpful to anyone, but there it is.