I realise that you are discussing a mystery on a frame found in Australia and am only speaking based on my Canadian experience. Typically, Canadian Chevrolet CMP chassis are not stamped with a serial number (some could have been but I haven't seen any, nor have I seen documents to say they should be stamped) but the "S" stamp can be present. This makes me think the number on the top of the rail and the "S" on the side of the rail are not related. The "S" in the middle of the serial is not something I have seen on Canadian trucks, which fits the suggestion of this version signifying Sydney assembly. There has been past discussion that the "S" on the side of the frame rail near the spring hanger might be an inspector's acceptance mark (possibly signifying "satisfactory?).
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