Yes, it's the left side dust cover, you have installed it correctly in your photograph. It tessellates with the dark grey ring on the cone, creating a 'labyrinth' seal to keep dust out.

I don't see it on the exploded parts diagram, (its right hand counterpart is visible, but included on part 17 not individually). I believe this is because Shimano don't advise removing it for servicing, because it is delicate and gets distorted on removal, risking drag from a deformed part. Obviously it is hard to clean in there without taking it off!

You appear to have kept it in good shape and put it back in correctly, so no harm done, good job.

Yes, it's the left side dust cover, you have installed it correctly in your photograph, into the left side of the hub. It should remain as pictured so it can tessellate with the dark grey ring on the cone, creating a 'labyrinth' seal to keep dust out. If it was reversed, then it would obstruct the assembly.

I don't see it on the exploded parts diagram, I believe this is because Shimano don't advise removing it for servicing, because it is delicate and gets distorted on removal, risking drag from a deformed part. Obviously it is hard to clean in there without taking it off!

You appear to have kept it in good shape and put it back in correctly, so no harm done, good job.