This is used to attach stackmat timers to third party mats. It's compatible with Gen3, Gen4, and Gen 5 timers (you have to space it differently for the Gen5 but it works fine). 

Instructions:

You create 2 holes (spaced the correct distance apart) in your mat for the latches to attach onto, then you screw the two pieces together. If the fit is too tight, then decrease the size of part B, if the fit is too loose, then increase the size of part B.