I meant in 3 for 6x0, like asked in 2 about 750, whether it displayed as 6x0 (O6xx/O600/O650) in Updater and/or on device screen during boot. Because you didn't mention otherwise, I guess it didn't display any model series on the device screen.

Let's next try flashing this O6xx [Only registered and activated users can see links. ] now that we know the device is reacting to it's correct 1651 HWID and it definitely has original V5.60 O6xx fw_all data in region 14. Just attempt a normal preboot Updater.exe flash. After it flashes don't disconnect from the PC. Hopefully it will immediately enter MSM and be visible in Disk Management as an unformated or RAR partition and can be formatted properly with RMPrepUSB. If it doesn't happen, read on.

Only if the above doesn't work, use the the Modded Flash Kit with 1651 folder and have only this this cured fw_all [Only registered and activated users can see links. ] in the root (alongside the Garmin folder, not in it). Flash modded_boot.RGN in preboot to execute this command in Updater.txt:

xrgn,14,1:/Garmin/14.bin
reboot

Have the PC ready with Disk Management open and a USB cable connected to it before you start. When Updater reports success, hopefully the device won't try to boot at all so quickly disconnect and reconnect it to PC then refresh or rescan Disk Management. Reformat it if it shows.