I agree with Don. My source was a tour guide at the U.S. Mint in Denver. Fairly uininteresting tour, but he tried to spice it up with Q&A time. One question was when milled edges started, and his answer was in England in the 1600's.
I've had tour guides acknowledge to me that they often make up plausible answers to frequently asked questions when they have no information about the subject. Assuming that Doug didn't do that, I'd be inclined to trust his sources over a tour guide's tale.