Either you select a P-No material that is as close a match to your unassigned base material or you review the original heat treatment performed on the unassigned base material and PWHT at 50 deg F below the original tempering temperature.
For the PWHT parameters, you need to review ASME VIII, B31.1, B31.3, etc. - not ASME IX. Compare your non-listed material with those in UICS-56, etc. vs carbon equivalents.