John - The adjustable elbow connected to the water heater draft hood in the first photo in the original thread is single wall.
The adjustable elbow at the bottom of the second photo in the original thread with the red arrow pointing to it is single wall.
How can you tell? That's what it looks like. See the crimps at the top of the elbow in the first photo? And they're much narrower than the b-bents they're connected too? Maybe Bob can be more specific. Heck, I know Bob can be more specific.