General fitting question

[ihscoutlover]

I own a 1997 K2500HD. I was looking at the radiator the other day and found a blocked off fitting below the cap, and overflow hose(see photo). Engine is a 5.7 Vortec. What is it for/go to?? Reason I'm asking is the temp never goes above 180 on the guage, I have a scent of antifreeze, the level keeps going down, but nothing keeps going up, and I've pressure tested all areas and haven't found any major leaks. Any help or guidance is appreciated. Also for some reason my phone keeps upside downing my photos, so my apologies.... Jeff