What is the thread size on the back of oil pressure gauge??

67Dart273 said
"Whatever this is" the gauge is built for the tube compression fitting to connect directly to the gauge

View attachment 1715092055

As Rusty would say: WTF is my damn crystal ball?

That right there is exactly what the back of my gauge looks like.
So I guess I need to find a way to go from 1/8" compression to the an fitting