Polish out the cracks and retest. Maybe surface imperfections. The raise metal around the bolt holes are from the lock washers biting in. See that all the time in soft metal. Some of the things you are seeing is found in castings. It's a sand casted piece. The mold wasn't up to temp when they poured the aluminum. So you are seeing incomplete filling of the mold. It shows up as voids and folds in the webbing on the inside. It does look like cracking on the inside of the cover around the stud hole. That's where most of the covers break at. Interested in the area around the plug hole. Looks like crack, but stops before the machined area around the threads.
Degrees in welding engineering and mechanical engineering. Have done NDT testing myself.
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