Ralph -
I'm wondering if that is a matter of semantics or local terminology ??
A couple points to be considered - the local fishing reports refer to gray drakes, Jim Scholmeyer in his "Hatch Guide For Western Streams" does not even discuss "slate drakes", and least important, I'm not familiar with the name "slate drake" for this neck of the woods.
Whatever it is ( a gray drake out here and a slate drake in your area ?? ), the pix should still provide a model for size, proportion, some level of detail, and color.
John