There's a relation between SN and IMEI. To recover SN one can restore IMEI in PDM mode. So the instructions will be:
1. Find a way to get into PDM mode.
2. Use tutty to send commands to the phone.
3. Try writing IMEI using tutty.
3.1. Replace 000000000000000 with your IMEI and hit enter: at%imei=000000000000000
3.2. You should get OK otherwise it does not work.
4. If it succeeds then the SN will be recovered.
1. To get into PDM mode, use the attached FTM file. Remember I do not guarantee that it works harmlessly and effectively, so have created backup beforehand.
2. You may need to erase LUN5 partitions to make it possible writing IMEI in PDM mode so again create backup of LUN5 and erase it.