Mark and cross,
You are both right, I missed that detail!
The 3 bladed flight was flown with cut down IRB's with SC 1095 airfoils and titanium spars. The original 53D has NACA 0012 airfoils and aluminum spar blades. Using the newer blades gave better data for the real purpose of the test, to determine the performance for the newer Sikorsky designs.