This is an older Marking gauge, seems OK

But why is there a curved cutout in the face?

jan065 by Skip Albright, on Flickr

jan066 by Skip Albright, on Flickr

Maybe for marking round wood blanks? 

That is an odd design.  I've got a marking gauge with two arms coming out at a relatively shallow angle to rounded ends, clearly designed to mark against edges of a constant curve - concave or convex - within its usable range.  This one would work similarly, but only for stock up to the thickness of the depression in the face.  Odd.

Yes, what Bill said.  If you have ever tried to use a regular, flat faced marking gage on a curved surface, it's difficult to keep it tangent to the curve.
Joe B