Re: How to open a Mosler safe with combination?

I have a 60s era mosler gsa or cia safe with an S&G lock mechanism with no hole in the back cover for a change key and no 2nd index mark on the dial. Can the combo be changed?

Yes, but I'd recommend taking it to a professional. It's what's called a hand change lock. Inside the case are wheels, sort of like gears, that can be aligned in different ways to set the combination. I advise a professional because it's a precision mechanism with many small parts, and also there are certain combinations that should not be used due to mechanical limitations, called the forbidden zone. Lock and safe technicians are familiar with all of this.
This type of work is much cheaper if you take the safe, the door, or the lock to the locksmith rather than having them come to you. Look for a brick-and-mortar local locksmith that works on safes and vaults, ideally an old-school establishment.