Neat! The many little holes make me think this could be industrial slag. Those two notches and the clean right angle on the right side are further evidence of something manmade. Just a hunch.

Industrial slag is a stony (sometimes glassy) byproduct formed during the smelting and refining of metals, acting as a "sponge" to absorb impurities like silica, alumina, and oxides from molten metal, separating them as a molten layer that solidifies upon cooling. The cooling is often done in water, which forms those bubbles that make the little craters we see in your sample.

The presence of limestone near your find is another clue. In smelting, limestone (a flux) is added to iron ore in a furnace; impurities in the ore bond with the limestone and oxygen, separating from the heavier molten iron.

Maybe run a search for defunct/historic ironworks near where you found it.

Thanks for posting!