In reality, most li-ion batteries are longer than they are supposed to be, especially if they have a protection circuit. Like I have some 18650's that should be 65mm long and they are closer to 68mm long.
The Quark 2xCR123A lights use 17670's too or you can buy a bored out tube that will take 18650's.
What you have to remeber is that none of these cells are really made for flashlight, it's more that flashlights were made to fit cells that were availlable.
Certainly most of the development for 18650's was for them to be used in laptops, I wonder if that is still the focus of new research or if that is all going towards powering electric cars.
Anyhow , 17670 is often a direct replacement for two 16340 , and 18650 requires 18mm tubes ..
Some lights are 17mm as they were designed to accommodate two CR123A batteries .. So unless you have a lathe or some other way to bore out a body , many chose to simply run a 17670 instead .. [ Dual Li-ion can be problematic and single cells are much safer ]
The Surefire 6P or 6PD is sold incl. 2 * CR123A primary batteries. That is 2 * CR123A = 6.0V min.
The 6 stands for …… 6V. If you bore out the body, you can accommodate 1 * 18650 = 4.2V max.
The other way around is not always safe. Some drivers (etc) are intended to operate at 4.2V max.
6 Volt is almost 50% extra. Some equipment can survive that, others don’t.
But why, except with intention of back-up, would you replace rechargeable batteries with expensive primaries.