From main screen ['Where To?/View Map' page] press and hold the battery symbol in upper RH corner for ~6 seconds to enter hidden menu > press either 'next' or 'back' repeatedly to reach 'Version Information' page > Start Test > More and look at first line: Boot BLK Bxxxx-00 etc. The four numbers 'xxxx' represent your HWID. Alternatively, look in a backup or connect unit in MSM to PC. Browse to Garmin root folder, look for GarminDevice.xml, open it with a web browser or text reader and search for '<PartNumber>006-Bxxxx-00</PartNumber>' near beginning of file.