It doesn't matter which version you use. The reason for having the h300.hex file is so it can be patched with the Rockbox bootloader so that when you turn on your machine, instead of booting directly into the original firmware, you boot into Rockbox. As I understand it, there are no features from the original firmware that are needed by Rockbox other than bootloading. Once Rockbox loads it completely controls the machine; the original firmware is not used at all...as I understand it.