Hey... I was thinking [emoji848]
If they have a rivet machine set up for the vent on a 26” and another one for a 22” and so on...
think about mass production here... they would have a mount or frame that would hold the lid from underside to support it.. with that being said a 26” would be have a flatter support plate.
Now my idea is if a 22” lid is riveted on the 26” machine it might cause a flat dimple on a lid (like in the photo) that has a more pronounced curve to it.
This is my guess / idea that might cause this marking on the lids.
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