Very interesting! It appears that the eye is somewhat collapsed and has lost considerable material on one side. Direct-on views of the more intact side might help. It is absolutely amazing how much appears to be intact, but this is not unique, and most of the axes I have seen have suffered sufficient corrosion that similar decoration would have been lost or obscured. Any hint from the salesroom about where this was originally found? I'll look for something similar in the references I have.
Thanks to the Internet Archive for a pdf of Peter Paulsen's
Axt und Kreuz (1939):
https://archive.org/details/AxtundKreuz
Though present throughout, look first at decorations on the pdf pages 36 to 40 (original pages 68 - 77)