It looks like it could be a lash egg which is a product of internal laying, but I haven't seen one inside a shell before. Kind of strange. Here is a link or two about lash eggs:
https://www.backyardchickens.com/t/637090/my-first-lash-egg-experience
https://www.backyardchickens.com/t/473989/weird-egg-weird-clump-lash-graphic-pics
https://www.backyardchickens.com/t/637090/my-first-lash-egg-experience
https://www.backyardchickens.com/t/473989/weird-egg-weird-clump-lash-graphic-pics