Makes sense to me. They say about 2lbs per bird per year. I maintain a flock of 130(ish) layers so they aren't eating some absurd amount of it or anything.
I was definitely slightly concerned about that when I first got the idea. I've thought about rinsing it somehow, but it seems like too much work. I always let it drain of any sea water and air dry, but I'm guessing a good bit of the salts stay with it in that case. They gobble the stuff up way...
We use oyster/periwinkle shells that I collect from shell bars during low tide from the ocean near my house, but we are part of a lucky few that has access to that resource. I'm sure there is organic labeled oyster shell at some feed stores. I wouldn't be surprised if any and all oyster shell...
It's pretty normal to see this kind of deformity. Definitely not something to worry about. I see it alot on my older hens. Some free choice calcium can help if that's the problem, but more often than not from my experience, it's an old age situation.