A Clark C series airfoil would be early in the series...
A 3-letter designation is almost certainly incorrect.
Looking at two photos of the wing root of G-23s in the Squadron Signal publication on Grumman biplanes, it's most likely a YH... rounding down from the leading edge..somewhat flattish on the bottom in the middle, kicking up a bit to the trailing edge.
Any upper reflex at the trailing edge isn't obvious in the photos.