TONMO, I've realized the pattern on the two fossils is that of "Beekite". Not having a lot of experience with silicified cephalopod fossils around here, I got out the acid bottle, fizz on the matrix, no fizz on the fossil. So instead of a Calcite replacement, they are replaced with Silica. Beekite may be a mineral, but the word is more commonly used as a descriptive term for the concentric rings of the replacement material.
The fossil from the first post showing the "Beekite Rings".