It has an SAE type 2 integral flange, looks like the two position option was available on different (later) models, but that might narrow the search of what type of props could have been used.
Propeller - Sensenich, two-position controllable, hubs C-2FM-3, C-2FM-4, or C-2FM-5 with PC-276A6 blades.
This prop has wood blades and an AD note which requires (I believe) annual inspections by a prop shop. Basically they have been AD'd out of service.
They were approved on the PA-12.
A thought, if you are experimental, it may be possible to operate certain compatible constant speed props on this engine. The prop will only travel to the stops in each direction and will move slowly. This obviously would require some research to find the correct prop.