I know this is a late reply, but the LTL of the 15500 is 50.5mm and the LTL of the 15400 is 51.5mm, both rep and gen. You can also see on the comparison pics there is a larger gap between bezel and the adjacent case line in the 15400. So yes the 15500 is a tad thicker but it makes up for this by having a slightly smaller footprint.
Thank you for taking the time to measure them. This kind of information is useful for both gen and rep AP owners (or owners to be)