Back in the day in many "starter" sets they included a DW, and it was called a "Dual Wedge" and was intended as a cross between a standard Pitching Wedge and a Sand Wedge (merely a wider soled PW). But, as with any dual purpose implement, didn't do either very well.
Additional Details: I had one long ago, and it is NOT similar to a modern gap wedge in any way shape or form.
DW stands for dual wedge. It is similar to a modern day gap wedge.