Thanks namisgr. I have a few of that style slab and the inner and outer wells seem snugly in contact all over. If your book here is the same, then the fact that the rings are in one small spot suggests contamination in that spot, wouldn't you say? Maybe some grease, something like that. If so, that would indicate sound materials, with the rings the result of handling issues.
Where is the first or second gen book example that is covered in them, as so many of the 3rd gens are? It doesn't exist, does it.
Does it?