The taper/width of the blades at the root end on all blades is for engine cooling. i.e. an air cooled radial engine requires more cooling air at the root to go over the cylinders, than a water cooled engine with radiators in a different location. So if your blades have less blade at the root it could be for an engine not needing as much cooling. There would be a specific propeller/test club for each engine model/series.