Thanks for the welcome.
Yes EC, using the regular (not pro) M Frame that I got last week.
Tom mentions that he thinks it might be change of lens design came with the 2nd generation of M Frame, but if that’s true, then there has to have been yet another lens design change after that. The 2nd generation came out in ’94 and the Pro came out in ’97. The lens from the Pro (purchased in the late 90s) doesn’t fit well with the latest M Frames that came out in ’99. So for a lens to fit in the first Pro M Frames that came out and yet, not fit the regular M Frame that came out just a few years later, a change in lens design must have happened with the new M Frame in ’99. And like EC said, since both take the same replacement lenses, when the ‘new’ ’99 M Frame came out, any changes to that one concerning lenses, had to also have happened to the Pro version.
I looked around and I found this review from triathlete online (Sunglass Buyer's Guide 2000)…
Oakley New M Frame $100
A tried and true classic, Oakley took their Pro M Frame and made them just a bit better, adding hinges and a slope for a tighter fit across the brow. The New M was redesigned and reformulated for even greater durability, and the Polaric Ellipsoid lens geometry means you'll get some lenses with the lowest distortion on the market, not to mention the safest: the shatterproof Oakley lens is the one you want to be wearing should you get in the way of an inflight bullet. Oakley's three-point fit is still very much in practice, and with various lens shapes to choose from, the New M can be tailored to your liking. Our tester found the headache-proof Hammerfang ear stems quite comfortable, as they can accommodate any head size without touching the temples, but rather behind and above the ears. It's a ton of features that should be no shock coming from such a sport frame powerhouse. For more information, visit www.oakley.com or call (800) 403-7449.
Sounds like a perfect time to make changes to the lenses too.