Having one of the old urethane Hammers that would flare wasn't totally unheard of, even if we didn't fully understand it yet. I myself had a red solid with a 2 inch pinout that I drilled with a basic label drilling. With the pinout though, it ended up almost exactly 3 3/8 inch from my pap, and the ball would flare 2-3 inches! We didn't fully understand what that meant, or why it did that, but MAN that ball would hook AND hit.
My guess is that it was one of those somewhat rare pinout balls. I've heard that Mr. Webb liked to experiment with different drillings, and may have just stumbled into a combination that created enough differential to cause significant flare.
I believe the original Hammer core had .024 differential and, if you used a drilling that enhanced it some, getting it to flare really wouldn't be all that hard, especially given that they didn't use nearly the volume or viscosity of conditioners we use today.
If the pin was out a couple of inches, and he put it on a line from his pap to his ring finger, seeing it flare wouldn't be surprising at all, not with today's knowledge.
