The b&w pic showing layers are actually cross-sectional pics of the layers of 0.4% and 0.6%-0.7% carbon steels.
Mr Wagner's primary sources of info with regards to this weapon were from scientific archaeological/metallurgical publications in China.
In my readings of several books (including Wagner's), I have come to understand that in the beginning, swords were forged and folded (resulting in the creation of multiple layers of steel) during the late Spring and Autumn/early Warring States periods, and then by the early part of the Han Dynasty, were forge-folded and differentially heat-treated by 2nd Century BC, to progressively result in the "hundred refinings" process by the 1st century AD, during the later part of the Han Dynasty, where by then, steel swords were typically of forged and folded stock, with forge-welded laminated construction (sanmai, inserted edge etc), and differentially heat-treated. These key processes were to endure for the next 2 millenia......
As for clay-defined differential heat-treatment, it was developed sometime between 200BC to 500 AD; the ridged-cross-section was most likely developed latest by around 500-550AD....