That is definitely a developing egg, and I see veins!
The tilted/slanted air cell kind of concerns me tho, all my eggs' air cells are normally centered at the very top. Could have been the way the Hen laid it, could be from shipping, hard to say.
Good luck with your nest hunting, hope you find a BIG pile of eggs. How many Guineas are in your flock, and what colors do you have?

Good luck with your nest hunting, hope you find a BIG pile of eggs. How many Guineas are in your flock, and what colors do you have?