Just a wild guess....
Reference Blade Radius ... refers to the propeller reference blade radius in an assembled propeller; e.g., 30-inch radius. A measurement from the propeller hub centerline to a point on a blade, used for blade angle measurement in an assembled propeller. A yellow adhesive stripe (blade angle reference tape CM160) is usually located at the reference blade radius location. (Note: do not confuse reference blade radius with blade station; they may not originate at the same point
From Hartzell STANDARD PRACTICES MANUAL 202A VOLUME 7