Ok, I'll help you with an SD pack to try flashing back to 6xx HWID 0596 but you must accept that further manipulation of this confused device might end with it hard-bricked, i.e. totally dead. First though, lets make some tests to be absolutely sure it's not seen in preboot by the PC. Be aware that 3xx & 6xx devices take only a few seconds to go into preboot and they stay in preboot for ~30 seconds only. By waiting too long you may have actually missed it. So first open Window's Device Manager and leave it visible on your desktop, then load the ORIGNAL 3xx HWID 0566 V2.30 RGN file to Updater.exe and select the radio button for 'USB Device'. Leave Updater's GUI visible on your desktop. Now hold the nuvi's power button down for only ~5 seconds before you connect it to USB (use a rear PC port directly, no hub and have the cable already plugged in to the PC before you run updater, start nuvi and connect). Listen for a Windows device connect sound and watch Device Manager to see if the nuvi appears. Hopefully you'll see this:

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Report back what happens. If it's seen by Updater and Device Manager then flash the original firmware. Likely it won't be seen though. Remember, all this is at your risk.