Measuring a blade against a master blade makes much sense, so now I understand that all blades of a heli type are equal and therefore interchangeable. That makes good sense. I have never been involved in helicopter design, so this is new to me. I always asumed they whirled the disc of each heli for balance, but that would not make the blades from other heli's interchangeable. I'll google a whirl tower, sounds like an interesting piece of equipment. Even at my age I keep learning.
Thanks for the information.