I’d like to tell you where I got these photos but then I’d have to kill you and that would be bad for business. 😀
They are posted with permission from someone who’s spending A LOT of time looking at fire doors lately.
And yes, these are fire doors:
And in case you’re not sure what you’re looking at here, it’s a view from above showing the section of the door’s top channel that’s missing, and the thru-bolts for the hinge in the void.