Well, perhaps we are crossed on terms. The "C" does seem to be incised and even in taking the first closeup picture of reverse, from the picture it appears that the margin or edge of the wing-like shape looks to be as well with discolouration on the inside of the wing being what looks to be the residue.
The obverse shape(s) really IMO appear to be localized collections of residue and/or oxidations of that residue. Many dilute dips can do that sort of thing if allowed to dry or desiccate.
Still, you have the coin in hand, not us. Interesting, this series was the core of my collection at one point. The 1929 and 1934 dates are the only ones not struck in proof..