To be honest as said in the thread you linked to I also think it is a waste of money.
As once you set the LPR on an Angel you leave it alone...if it isn't performing then you check it with a gauge and then adjust as required (doesn't need doing very often)
The side plug is said to be for a gauge...which is needed as you will have no idea of where you are psi wise when you start to move it as far as a gauge attached well all ok if it doesn't foul on the barrel (it will depend on the threads if they are precision cut to always line up the same time) but then why have something like that hanging on the front of an Angel...if I wanted that I would buy an Impulse
Of course on the later Angels you do use volumisers but they are streamlined not with holes to fill with paint if hit.