@Gazzabear Thanks for contacting us. I would be happy to clarify this for you. Both of these memory modules have 204 pins, as do all DDR3 SODIMM memory modules. The pin count is important to the memory fitting into your system. The difference in specifications between these parts is the density. The CT51264BF160BJ is a higher density part and only has 8 little black chips on one side, and CT51264BF160B is low density with 16 little black chips total. More info on density can be found here. Your system appears to be compatible with either high or low density memory, so I would just purchase the lower priced memory if there is a price difference. Please let us know if you have any other questions or concerns.