I was suspicious when you said it was cast aluminum and not stamped… I zoomed in to try and spot what I suspected and I think it’s pretty clear. I bet you there’s an outer shell and an inner shell, they (partially) fill the outer shell with molten aluminum then push the inner shell down to squeeze it out until what you have is s as middle layer of cast aluminum. You can see the outer and inner shell on the lip to the right of the “open” word, look for a line near the top of the rim and a line near the bottom. The inner shell is effectively perpendicular to the cut by the time it comes all the way to the outer edge of the lip, so it can appear thinner than the outer shell which may have been cut at an angle.

To see if those shells are steel is easy: are they magnetic?

It would be good if they are, you don’t want aluminum in your food and it does leech extensively especially in acidic foods.