The best way for Drive Series is the method "1" described in that thread. Connect your DS50 to a computer (or just browse a backup if you have one) and look for the file named GarminDevice.xlm in the device's Garmin folder. Open the XML in a text reader or web browser and look for "PartNumber>006-Bxxxx-00</PartNumber>". The four numerals after B are your HWID.

RGN_Tool and Updater respectively convert the GCD file to an RGN file and allow that firmware RGN to be flashed to the device in preboot. However the fw_all.bin (main system firmware) component of the file needs to be modified/patched using an application called "Universal Firmware Patcher" so that the device will accept unlocked detail maps for areas other than it's original factory entitlement. Search for that name or by it's alternate name "IMGPatcher" but ensure you get latest V2.20.

If you've done fw patching before with GPSMAP64 you'll be able to do it again with the DS, the difference is only that the 64 may not have had to be initially flashed by RGN in preboot but could be done with a GCD file even initially. For your DS50 with GVS/GSV protection you must flash initially with an RGN in preboot mode, later updating method is the same with both devices simply using a patched GCD file.