Egg industry does what they call "candling", passing the egg in front of a strong light. Hens will often have a spot included in the egg, a piece of tissue, no harm or danger, fine to eat. When the industry has those the sell them to bakeries and other high end users.
I had one hen, (yep, Henrietta) who consistently laid an egg with a spot.
Farm eggs, on the other hand, are not candled and there is no way of knowing what's inside. From double yolks, to spots and a few other extreme rare choice things.
Should the egg be 'rotten' it was probably lost somewhere in the hay/nest/wherever during a hot summer. I've never found one, including a nest of over 15 eggs that a sneak away hen was laying. Have no idea how old they were but wasn't going to try to find out if any were OK.
taxy pretty sure it was a farm egg, even in the 60's I believe they candled. But a good lesson as to why!