In the Phylogeny of humans and reptiles, what is/are some of the most recent common ancestors?
I think that you are looking for the amniote common ancestor.
Amniotes are the group of organisms that have an amnion, a specific membrane around the egg, among other features. This includes reptiles, mammals, etc. but excludes amphibians and fish as indicated in your tree.
The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago, in the Carboniferous geologic period. Amniotes spread around Earth's land and became the dominant land vertebrates. They soon diverged into synapsids and sauropsids, which persist today. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).
That citation 7 points to this paper, which used paleontological data to assign time estimates to various diversification events inferred from molecular phylogenies; this allowed them to approximately estimate the time of emergence of amniotes from the amphibian clade.