An ordinary A/C gauge set like used on central a/c for years will connect to at least the Low Side Port.
At some point, at least some carmakers downsized the High Side Port's size. This kept people from connecting those "recharge kits" (can of R12, piercing valve and a short hose) to the High Side and bursting the can.
So, connecting to the High Side with ordinary gauges required this:
The male thread you can see fits the gauge set and is same as the Low Side Port. Inside the end you cannot see is the smaller female thread for the High Side.
I found this item on AutoZone.
