Ok, the original blue steel square backed grip frames only on the Super Blackhawk, 44 mag. Then along came the brass grip frames,, and they were put on other models. Next came the 357 Maximum in blue steel.
If the gun in question is a 357 magnum,, and has a blued grip frame,, it has been swapped. If it has a brass grip frame,, it's improbable but not impossible to have come that way. A factory letter about a brass one would be in order.
Grip frame swaps are not uncommon.