In some of the AR-88 handbooks on line is a sheet for adding a meter to a receiver without one. I am pretty sure it gives the dimensions.  My AR-88 is an F.  This is the version used in the triple diversity rig.  One of the mods is an IF gain control to equalize the gain of the three receivers. It serves no purpose in a stand alone receiver but is in the right place to drive the signal meter. You need a trimmer for the meter.  On mine the signal indication is surprisingly  accurate.  The F has a couple of other simple mods to the AVC which are easily undone and result in smoother operation for a stand alone receiver.