Bill Hayton's book The South China Sea documents how very recent the PRCs claims to the SCS are. The first actual military move by PRC forces was against some tiny islands in 1955 held by KMT forces. That move was probably more about Taiwan and the KMT than the SCS. No Chinese maps even showed the SCS until 1935 and that was an obscure map maker, nothing by the KMT. The earliest "documented" Chinese claim was the nine-dot line which was fabricated out of thin air by the KMT.