Thanks for the information. The surface is smooth on all sides, I know the pictures don't help show that. What really caught my eye was the areas where a handle may have been affixed. The top, bottom, and rear (the thickest part of the stone) all have distinguished raised "build up"?
I appreciate the response and help with what to look into, thank you!
I think you have a natural concretion, not an artefact. It looks to be heavily weathered iron-rich sandstone of a kind sometimes just generically called 'ironstone', which can contain a mixture of iron minerals (goethite, hematite, siderite etc) and frequently has a distinct cortex arising from differential weathering. I would believe the distinction between cortex and interior is what you are perceiving. People tend to think of concretions as being round, not flat, but they can be any shape. Something like this: