I haven't tried all the Oyster Shell on the market but that Manna Pro one that says it's "Oyster Shell and Coral" is really just coral and the hens don't like it.
The picture on the Purina bag is discouraging as it looks like white rocks.
It appears "Oyster Shell" now has nothing to do with Oysters and just means "Source of Calcium" no matter how inferior. The liars!
I've scoured the forum through threads on Oyster Shell trying to find out where I can get the right thing (FLAKED Oyster Shell) for a decent price, but even one highly recommended brand, when I read the reviews online, has since started to use mixed Seashells razed from beaches rather than Oysters.
So, I looked at alternatives including "Crab meal" and the aforementioned Seashell but I have trouble making decisions when it's not what I sought.
Gaaaaaar!