What's the biggest difference between SRG3900 and SRG3500? available bandsplits, naturally (3900 offers UW-band and thus access to amateur, where as the 3500-series does not). Yes, there is TG, but someone sold a slice of our 70cm band years ago, so it starts at 434, not 432.
Also, if a radio has been programmed with RM2 and upgraded to the latest FW, can it be downgraded to a FW compatible with RM1.8 and then use RM1.8?

Furthermore, what's the latest FW that RM1.8 supports?