168s is the battery that will fit the tube. And you do want to make sure it has the right circuit in it for best use.
The batteries with the "A" are 19mm in diameter. The only lights they fit are the Pila and Wolf-eyes line, Surefires U2, and some of the custom jobs such as the LionHeart. The "S" batteries are 17mm in diameter. The "S" by the way stands for Surefire. The 168 is about 2x123 in length where to 150 is 1.5x123 in lenght.
Pila "S" batteries are actually a little bit fatter than 123s. They do fit in the tubes of most 123 powered lights but there are exceptions. The A2 and L2 are more narrow than the other Surefire lights when it comes to internal size if the battery compartment so the Pila batteries will not fit.