what do you call this feature?

[VTsnowbird]

The wall between the chimneys on each end. Almost always symmetrical, same on both ends. And is it just a design feature, or does it help brace the chimneys? (Image from Library of Congress)

[K'ledgeBldr]

That would be considered a “parapet wall”.

It closes off the gable end of the roof, and may add some structural integrity to the chimneys-

If you look closely at the rest of the rake of the roof, you’ll see parapet wall there also- but at a different slope than that of the roof.

The parapet walls also have “anchors”- the iron “S” were used to help hold the brick wall to the framing of the structure.