I'm glad you mentioned that. I still have some old AW 1100mAh P17500's I use in the M3's. I just checked and the 1200mAh Keepowers I have fit, but do not function in my M3 either. I compared the cells and they are a few MM longer than the old AW cells when you line two up and compare them to the old cells. I guess if I have to find replacements just to use in the M3 I might have to get some Trustfire 17500's, or see if I can find something that is 16500 in size if they make such a thing. I'd rather have the fatter 17mm cells though because of reliability/rattle concerns.
The Keeppowers work in my C3's as well. I wasn't expecting them not to work in the M3. They've been fudging the diameters and lengths of these cells for years now to the point where the specs are just marginally accurate. Sometimes it's hard to find the right one. Unfortunately there are only a few 17500 cells available these days, which is odd because they used to be one of the most popular sizes. And 18500's usually only work in bored out Surefire lights or ones designed for the size.