Thanks for the replies.The Streamside CR123A batteries themselves are marked 3 volts. So the battery size mentioned may be too much. I just started getting interested in flashlights and I'm not sure what I can use this kind of application.
The individual cells are marked three volts, but the light puts them in series, so they present six volts to the driver circuitry. Since an LED's forward voltage (the voltage needed to light up) is three+ volts, the driver circuit is most likely a buck circuit, that is one that is designed to output a lower voltage than the one input. Depending on how the circuit is designed, the lower input voltage from a single LiIon cell may be dropped below the forward voltage of the LEDs, which means no light. On the other hend, since there are no modes available to the TT 2L, just various combinations of LEDs being turned on, there may not be a driver circuit at all, in which case the single LiIon cell should work just fine. The only way to find out for sure is to try it.