Why you get a shock?
A relay is an electro-mechanical switch that opens and closes under the control of another electric circuit.
When current flows through the coil of the relay, a magetic field is created that causes an armature to move, either
making or breaking an electrical connection. When current is removed from the relay coil, the armature returns to
its rest position. It is important to place a diode across the coil of the relay because a spike of voltage is generated
when the current is removed from the coil due to the collapse of the magnetic field.
The pin in the end of the collective grounds the primary coil of the "Ground Relay 2CX".
As the field within the relay collapses (through your arm) you get a taste of the spike. Ouch!
I have personal experience and it definitely bites.
32-60-03 Ground Relay
PP20E -> CB 1CX GROUND REL -> Relay 2CX -> 8VE COLLECTIVE PIL
It controls various things through TB27 A,T,J and V respectively.
Maybe 2p worth of diode may fix it!
Needles to say that the radios may endure the spike as well?