if you planning to purchase the light from DX, I recommend you to read the comment on the product page.
Some users who already bought the light, might already reported it, if that specific product have problem.
I have the ultrafire C3 Q5 with extension tube and WF-501B R2
they are great lights for the $$
you might need to put more lube on the thread.
on the ultrafire C3, I had to resolder the connection on the head that connecting the (-) to the body.