It's either a very tight fit - may take a 17670 in that case, or it draws too much current. I'd guess, since it is intended to be used with 2xCR123 that the tube is too narrow to take an 18650. Or it actually needs more than 4V to emit light.Pity it's a Q3 emitter and not an R2 or better.
After long waiting, I canceled order from KD and eventually ordered ZF7371 from here:
I paid $7.38 for shipping to Croatia, and the package arrived 2 weeks later.
I agree with everything said in the review on MesserForum.net. The beam is really nice, without any rings, thanks to heavely textured reflector.
However, I figured out that I can improve a few things. First, I replaced the original glass lens (31.4 mm diameter) with 30 mm AR coated one from KD. To prevent loose fit, I added another o-ring. When the bezel is tighten, this o-ring fills the gap between the body and the lens, and gives an additional water tightness.
Another modification is stailess steel ring (made from M22 bolt), which purpose is to prevent rolling on a flat surface, and enable using of the lanyard.
The ring is glued in place with a high quality UHU ENDFEST 300 epoxy.
I also painted red dot on the selector ring, to improve visibility.
Interesting, my specimen is without Ultrafire name and the model designation. Finish appears to be HA-III.