This is bound to be a relatively high quality tire. 65 psi would certainly not be an issue with a 1 3/8" tire.

I would be surprized if you clean both tires really well that you will not find the tire pressure range molded into the sidewall of the tire. Usually it will have Min 40 Max 65 PSI or something like that actually molded in the tire. I believe it is probably required by law in many countries.