That's very odd because MYS6 is the firmware I flashfixed to cross flash my SHM-165P6S to DW-G120A with no problem. It did say something about size didn't match or something (don't recall exact message) running flashfix, but the process was continued and finished flash successfully.

Edit: Post #9 in your link also seems to indicate it's possible to flashfix even MSOR unless there's something I'm missing there. Is it possible there've been some updates to flashfix since last tried by mciahel?