Thanks to #Giomen in [Only registered and activated users can see links. ],

and to his file for HWID 2077, version 4.60

[Only registered and activated users can see links. ]

I have finally managed to roll back the firmware of my nuvi 2597 to 4.60.

But... Unfortunately my device (Nuvi 2597) really does not allow for other maps than the one from Lifetime update, through Garmin Express...

I've fished out more info:

from #catymag' post, on 9th February 2015, 02:02 PM:

Unfortunately as stated in OP quote

New gimgunlock-0.03+MapSigPatch
unlocks NT (not NTU) IMGs in newer devices, but not working with all new devices 2xx9
If you have 2xx7 (8) (PartNumber> 006-B1582-00), works

If you have 2xx7 (8) (PartNumber> 006-B2077-00), doesn't work. [THAT'S MY CASE]

Your device have a new protection in firmware.

And also from #Cents:

I am a new member and a complete Newb at updating maps in a GPS. I just bought a Garmin Nuvi 2597 from Costco Canada a week and 1/2 ago. Right after I got it I used Garmin Express to updated the maps and a software update (I think a firmware update but not completely sure). I have read a large part of this thread and others and have checked the Garmin XML file and it has the dreaded "PartNumber> 006-B2077-00" near the beginning of the file. Is the part number tied to the physical device and so is unchangeable and I am simply done until someone finds another way around this with a new gimgunlock or similar, or is the part number tied to a firmware update that I might have done, and if I roll back the firmware based on info in this forum and use the latest gimgunlock I could then install a Garmin European NT map?

Answer by #catymag: @Cents.
Sorry actually you cannot unlock maps with unlockers in your device.

So, I thank a lot for the help - it's a great forum - and I'll be waiting till some genius can figure out a way to remove this protection from Nuvis 2597 (and the others) with part-number "HWID 2077"

Cheers!