When the freeze plugs were installed at factory, was any sealer used?
Would have to look at the original engine build paperwork. If so you would not be able to see it on the fully assembled and painted engine
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As to the other part of the subject
To expound on Bob's comment about competing in classes other than SAAC Premiere or MCA Throughbred or an unrestored class one way of "getting by" is to fill or remove a marking before you paint the engine. It will look as if the paint filled in it during the paint job
If your going for originality often painted did a single pass with the spray gun leaving bare metal, shadows or thin paint in pocket areas. Of course there are those times and examples where the worker would coat the engine more.
Most visible are the two on the front surfaces of the heads pointing forward