Unit ID is not in reg 41,I know. If you erase 41 Unit ID wont be erased. I though he flashed something else after that to restore model n.and nuvi lost Unit ID.
But better not to expain more here.
No way to readd codes and registration date, I have the same in my 3790, but no problem with myGarmin and updates.
I lost model n and I couldn't update with my Lifetime at myGarmin, MU etc. I restored model n. and all returned ok because infos are stored in Garmin's records .
This happened before we knew how to backup NV, in first attempts in 37xx to 34xx conversion ,so I lost codes forever.
Kunix sent me a program to restore them but failed. Probably it works with 1XXX series.
Loosing Unit ID is another story a bit more complicated and, as I know, service cable is needed if you don't have backup.