I guess the WF717 Fluid Head Standard 3/8" Mount was out of stock for a few months. The 3/8" standard mount (not bowl mount) is what is used for tripods with the extendable center column (neck). It might not be the most fluid head available, but the low price and the fact that it's made mostly of metal makes it very popular for use on top of Sliders and Table Dollies. It can also carry a decent amount of weight and is used to support small small Jibs / Cranes, or under Sliders.

The WF717 standard 3/8" mount is Available via Amazon (Click Here)

WF717 Fluid Head Standard (not Bowl) 3/8" Base Mount

And also available via eBay (Click Here).

WF717 Fluid Head Standard (not Bowl) 3/8" Base Mount