I realize this is quite late. Did your egg hatch? I had one just like that, was told it was a saddle shaped air cell. It hatched fine. I have another on my set that's currently on day 8. I have some funky air cells going on right now with this whole set and I'm wondering if it has to do with my hens having been wormed with Wazine recently (I tossed 73 eggs over the two week withdrawal period, ugh that was tough!). Two had air cells smack dab on the side of the egg, those have since grown upward, but now one of those and one of my other's air cell seems like I can see liquid at the bottom moving or coming into the air cell when I tip the egg. Or as if the air cell is baggy rather than taught and I see the egg fluids going between the hanging bottom and the air cell and the shell. It's very weird.
I'm just going to let them do their thing as all have active embryos. Fingers crossed!
My menagerie: Buff Orpington chickens, Bronze and Bourbon Red Turkeys, Coturnix Quail, a Labrador, an American Pit Bull, two wild and crazy boys, a loving husband, and me!