What Are The Differences Between The Vandenhul Cartridge Families

It seems like VDH had an abrupt departure in his cartridge designs awhile back. There were the metal body ones like the Grasshopper and other types, and then all of a sudden the open architecture of the Stradivarius cartridges appeared. Seems like the specifications also indicate a fundamental shift in design philosophy also. Yet I believe there is one hold over, or at least the silver and gold versions of the Grasshopper stayed around longer than the other ones.

Does anyone know what changed, and what the corresponding differences in presentation might be? I was looking at 3 of the Frog series cartridges available here on Audiogon.