It is named the Pressure test fitting in the TO and I can't find any real info on using it. The old 1C-130B-2-10 shows the fitting, but the book is not specific about hooking it up. This was back in the day when knowledge was handed down. It says to use an air cart though.
"d. Start the MA-1A and pressurize the bleed air manifold."
Suffice it to say, it was probably put there for manufacturing purposes to check the aircraft before any power was put on it to run the GTC.