Ill post pictures of it in a second. It is VERY simple.
edit: The gauge is held in place by an oring and seals the rear portion of the backcap. The gauge can be clocked with a allen wrench by removing the backcap and turning it. Not the quickest way to do it but its the most elegant way. No eternal set screws to mess with.
It should also be noted that the new backcap has been opened up a bit to gain the volume lost by adding the gauge.