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You can go either way depending on what you are sharpening and the edge that you want.
For a razor, I'd do spine leading to draw out the edge just like on a pasted strop.
If you do edge leading...

It's a microtome finishing plate.
Usually had 4 powders with them: Tripoli, chromium oxide, ferric oxide, and carbon black that you mixed with glycerin to make a paste on the glass.
So it was used...