@ChacalCH
If you are trying to flash [Only registered and activated users can see links. ] to use unlocked maps with your 64? If so, you may very simply load via a patched GCD, there is not a need to flash in preboot with units that have GVS[GSV] showing as "---" in the fw_list, that's only for ones with "Yes". See [Only registered and activated users can see links. ]. Post #34 is also quoted in the initial post.
Spoiler: From fw_list

Model: GPSMAP 64 | HWID: 1859 | SWVS: 420 | MAP: Yes | GMA: Yes | MSV: Yes | GVS: --- | File: GPSMAP64_WebUpdater__420.gcd

Here is a patched gupdate.gcd with fw version increased to 421. [Only registered and activated users can see links. ]. Place the extracted patched gupdate.gcd on your unit, or load it with WebUpdater.