It is 51hp (or maybe 51.5). There is a metal frame that holds the rails which determines the width. So if you place modules on the sides that have a little space underneath their plate, like eg Disting or Fxdf, you can squeeze in 52hp. But I’m not sure if this goes for all pocket cases.
My other case from them is 78hp, but in reality is only 77.5hp. So you probably need to have the case infront of you to really plan how you set it up.