Neil, When it comes to real Garmin devices, I am more scared of them then this guy
I just had bad luck with these things hardware wise.
As for the rgn header, this is my limited understanding of it and could be wrong, the header is 0x3C bytes long and is followed by data. At offset 0x32 is the region number.
3 Things to edit
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0A 00 56 00 - Region data size + 0A (written in reverse)
0F - Region number (ie 15)
00 00 56 00 - Region data size (ie 5636096 bytes) written in reverse
XX XX XX XX actual region data
I just realized you can just use RGN Tools v0.84, to create rgn file, then just hex edit the Region number at offset 0x32.
Good luck mates.