I flashed my 3750 to 3450. Then, I flashed region 41 froma real 3490 and it became a 3490. Then I flashed to 3790 US. It worked but the GPS was still a 3490 (which was expected). I tried to flash CHN fw but got rebooting loop. I flashed by original region 41 from a 3750 to try to get back to original.
This was not enough. I dumped all regions and noticed region 34 was altered with a 1101 HWID that appeared in region 34. I flashed both original region 34 & 41 and now I am back to normal as a 3750 that can be flashed to CHN and other foreign firmware.
Conclusion: beware of a few more bytes in region 34. That could be the issue you had to switch to SgMy.
Normal dump for region 34 for 3750:
Offset 0 1 2 3 4 5 6 7 8 9 A B C D E F
00000000 XX XX XX XX FF FF FF FF FF FF FF FF 00 00 00 00 XXXX’’’’’’’’....
00000010 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF ’’’’’’’’’’’’’’’’
00000020 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF ’’’’’’’’’’’’’’’’
where XXXXXXXX is my gps ID. Extra bytes appear at offset 0x10-0x17.