Coolant Tank sight Plug Cummins

[Belize2000]

Looking for a coolant tank sight plug. Ours fell off.
It's just under 1/2" possible metric.
Not use of pitch, I tryed to get a pitch with no luck with standard or metric.
This is not the explanation tank but pressure tank made of aluminum, it's for side radiator.

1999 Gulf Stream Scenic Cruiser Motorcoach.
Cummins 330 ic