Was 6.57 the version that came on the watch originally? If so, you might be out of luck. All of us are only going to have alpha/beta firmwares saved on our computers, the public release versions can't be downloaded and saved manually. And Garmin doesn't distribute legacy firmwares to people. So you may be out of luck on this.
Oldest firmware I've got saved on my computer is 8.13 which I believe was the first beta firmware available, so it's probably the oldest version you'll be able to get.
