You might find this Sheldon Brown article helpful or at least interesting...

https://www.sheldonbrown.com/freewheels.html

And here are a couple threads here on BF about them....

Freewheel threading

Regina Freewheel

In the last thread above, post number 6 gives how Regina marked the type threading that was on their FW's.

Regina Freewheel

Since it's a Trek, I'd tend to give better odds that it's English threaded. And that is what became the standard for most FW's today. But I'm just guessing.