Steve,
I am not questioning the statement made by the seller about these cakes coming from India and being antique. It wery well could be. However, there are modern methods to produce them. See this:
New_Wootz
The process describes making wootz cakes using cast iron, the freshly crushed green glass, dandelions and iron powder. It also shows four of these cakes, too bad no close-ups available for visual comparison.
I do not know if 'new' cakes will look the same as yours, or whether they'd show the same wootz pattern upon etching, or how to tell if they're authentic. Perhaps Rick will agree to comment.