Found this small oddity while breaking apart limestone. The pitted appearance was interesting. The pits also seem to extend the whole way through. They also appear to wrap at a 90 degree angle on the side that isn't broken. The broken side reveals how they go through. I chipped away a little at the matrix, but didn't go too tough to keep from breaking it.
Whole specimen with scale: (stacked photo)
Showing outside 90 degree wrapped edge with same appearance: (stacked photo)
Broken edge showing channels going through the width.
Additional view of the top (unstacked photo)