Hi Peter,
Firstly the WA adaptor is made from some sort of alloy as opposed to plastic. This is because the thread that screws onto the lens filter thread is recessed into the WA adaptor as opposed to sitting proud on the Std adaptors. This allows the filter holder to sit about 5mm closer to the lens when using the WA adaptor.
This is done at the expense of an outer thread that would allow you to fit further screw on filters to the front of a Std adaptor but no such option exists when using the WA adaptor.
M.