Perhaps a silly question, but can anyone confirm my guess that this larger ring seal (copper color seal in the bag) in the overhaul kit is for the Allen head plug on the left side of the engine case?

I always have to refer to these when rebuilding. Click on the Legend link for the legend.

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The sealing ring for that should be a solid aluminum washer, not a crush washer

What is under that? I never took one out.

Secondary oil pressure relief for the lifters, plunger and spring.
Hydraulic bus and later replacement cases don't have this.