Help identifying exhaust bung

My 1983 242 Turbo down pipe has a 2 bungs. (Shown in the picture as the larger bung hole is the 02) Can't figure out how to plug the smaller bung.

Whats the second bung hole for? TP? EGT?

I lost the plug....I guess from engine vibrations the plug came loose and is now gone.
What thread pitch is it?
Where can I get a replacement?
In the meantime I used a bad red block temp sensor, same thread pitch, to plug up the leak.

1/2" npt?

The second bung is a test port for an exhaust gas analyzer. The proper way to test the mixture is before the cat.

You should be able to find a plug in the junk yard. They're on all 4 & 5 cylinder cars -'98.

Not much junk yards on Long Island NY...
Thanks for the tip though

Holy terrible welding batman