[Moonman62]

I'm thinking that must have been the vertical stabilizer, since there was no vertical stab visible in the "sideways" frame.

That would make sense if the orbiter had just turned sideways, putting a huge load on the vertical stab. Yesterday, NASA said they were concerned with the rate of correction of those yaw thrusters that had fired. They didn't say if it was overrate or underrate, though.

[snopercod]

That would make sense if the orbiter had just turned sideways, putting a huge load on the vertical stab

No (if that is what broke off), it did so before the orbiter turned sideways, which makes sense.