Today I was listing a Windsor on eBay from Stevens & Co., a small turn-of-the-century manufacturer. The frame has a very tight, hard feel compared to the almost spongy eyewire of some of the American Optical frames.
So this got me wondering why American Optical standard Windsors were not stiffer. After all, AO was the top manufacturer of the day and they didn't get there by making crappy frames.
Eventually I figured out that AO must have produced greatly flexible fronts so that multiple shapes could be mounted. We see round, P3, and oval shaped lenses in these spectacles all the time.