I have also a Nuvi 255 and i study this too. Differently to the my Dakota 20 this gps have the follovings rgn:
5, 14, 15, 16, 34, 41, 43, 45, 48, 86 and 91.
Differently to the Dakota 20, that have the internal flash disk in the rgn 15, the Nuvi 255 have it in rgn 48, and the disk image starts at offset 0 (in the dakota start at 0x520000).
The rgn 45 and 86 seems empty. The other regions have the same contents as all Garmin.
Rgn 91 are 512kb long and are used. Also rgn 15 are used.
What are the contents of regions 15 and 91?