I'm now satisfied with what I have come up with. Much speculation has been made over the years about the what the "S" signifies. Many claim it meant smokeless or staynless. Neither one makes any sense because smokeless was invented at the turn of the century. There was no need to make a special stamp on live rounds when smokeless rounds were by now the norm and was printed on the ammo boxes. Another claim such as Staynless was a particular branding by Winchester for Winchester ammo and was not associated with U.S. Cartridge Co. ammo, even though the company that owned Winchester by this time owned U.S. Cartridge Co. Staynless is also not feasible because the were many other different rounds made by U.S. Cartridge Co. that had the S stamp and were made of copper and other metals.
An S die could have just been chosen at random to do the year long control sample run or it could have meant just that, Sample? How about S for Sporting ammo? (Non military). All good theories, but from all the evidence I have looked at and from Goddard's own mouth is the simple answer that the S stamped on these bullets were merely a manufacturer's mark representing the U.S. Cartridge Co.
By the end of 1926, much of the U.S. Cartridge Co's ammo manufacturing machinery left Lowell Massachusetts and was moved to New Haven, Connecticut at the Winchester plant.