120 psi on 26x 1 3/8" tires?
 

I doubt the 26 x 1 3/8” version is rated at 120psi. (The 700C version, maybe.) I’ve never seen a 26 x 1 3/8” rim with hooked beads, which are needed for such a high pressure tire.

You are correct. Trek just confirmed that this size is rated from 70-100 psi. That still sounds a little bit strange for a 3-speed, though. I am not understanding what kind of bike this tire is designed for.

Just follow this:

https://axs.sram.com/guides/tire/pressure

Do it for safety.
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I like that calculator. I ran it for the 25c tubulars I ride. It gave me about 8 psi less than I like, no surprise. I don't care for the current ":squishy". But the front vs rear difference was the 5 psi my coaches taught me a million years ago and that I have been using ever since. Difference between wet and dry was 10 psi. I've never measured my drops to wet weather pressure but I probably do just about exactly that.

But - tubed clincher wasn't a choice? I don't think I am the only one who still riders them. (Should I run out and buy a lifetime supply of tubes? A nitrogen gas filled freezer?))

I only ride clinchers. For this calculator, I select the first of the four Rim Type options, "Tubes."

There are tandems and perhaps loaded tourers riding on 26x1-3/8" tires, somebody had to make tires suitable for them!

Indicates a strong tire casing, so likely durable.

While that guage does put one in the ball-park, it does not indicate any tolerances for a given tire size. When I used it, it said I should be using about 62 psi for my 26-inch tires. This is about ten pounds more than my present tires' recommendation, and they are from the 1970's, so I have to be careful. The Bontragers (reportedly) are 70 psi, and I do not know if I could go with 60 psi in them. Moreover, I want to ride with my present air-pressure, which is 50 pounds, because of the comfort.
I skipped the Bontragers and committed to some N.O.S. vintage 590's, which are Michelin, made in England white-walls. I just did not want a harsh, high-pressure ride. Thanks for the suggestions, Jeff and Surfer.

Sun offers the CR-18 in 590.