Posted the question on the VCCA and got a hit, and then did some research:
From my reading and reserch could be Manufacturer Stamp, "S" Being AO Smith, seem that were big into making Frames back in the day.
Really like the info in the PDF link below, Pretty interesting about the manufacturing and the depths they went to in automating the process !!
Some Interesting Info & Links:
AO Smith History Timeline AO Smith History Info PDF About the Mechanical Marvel & Frame Manufacturing Process 
