My pre 29 four screw came in a dark blue/black box serial number S181xxx, and shipped 1958. Although yours has an earlier number, I understand the 8 3/8's were introduced with the walnut pres cases, and not the blue/black box. The clamshell case came in around may 1960, maybe earlier, so if your gun shipped later than the serial number would project, then it could be original. A factory letter could nail it down.

Charlie