marker but why?

[skipskip]

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

[Yadda]

Maybe for marking round wood blanks? 

[Bill Houghton]

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.

[JoeCB]

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