Well, it may have something to do with the engines. We are all used to seeing the Lib with the turbo charged R-1830-41.
I think the aeroplane in the photograph is using a mechanically charged R-1830-33, the difference being noticeable by the shape of the engine cowling, the turbo unit cowlings being elliptical.
The early Libs also had Curtis electric blade pitch change motors in a longer hub, whilst later ones had Hamilton Hydromatic pitch change.
With this in mind, it may just be a back plate for a missing ‘spinner’.