What you called a spacer really look like the washer that is used on an flexplate (Automatic), and there it goes on the outside of the plate. The flexplate is much thinner, and needs this washer for support.
If this is a spacer like you say, I would think that it should be machined on both sides.
I have never seen a spacer on a flywheel, and think it should be avoided if possible.
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