First, to be clear, I do not dispute what Bill wrote. In fact, I believe that the elimination of the soft-fitting process was the cause that brought to an end the placement of serial numbers on the barrel flat and on the rear face of the cylinder.
What no one has ever explained to me is why, in some instances, those serial locations stopped sometime before May, 1957.
Here's why I ask that question: I have owned two K-38 Masterpiece revolvers that lacked the serial number on the barrel flat and on the cylinder. One of them shipped in November, 1956. The other shipped in January, 1957. I still own the former revolver. The other is now owned by a friend of mine. Notice that both of these revolvers actually left Springfield before the soft fitting process was eliminated. The two revolvers are about 7200 serial numbers apart. Both of them had to have been assembled in 1956. If only one of them was missing the requisite serial numbers, it could be a simple anomaly. But two of them? Something was going on. What?