If you know weights on axles , or better on seperate tires, I am able to calculate a save lowest pressure .
so its not important what the old advices where, yust give weights and speed used as max of vehicle, and of tires :1 maxload or loadindex :2 kind of tire ( think P tire in standard load or XL/reinforced/Extraload) and 3: speedcode of tire ( letter Q and above most used for radial tires ).
For old cars also often oversised tires where used if normal persons-car, for Trucks often yust enaugh maxload to carry the max allowed axle weights.
So weigh the car once , loaded as you mostly use it, and it will count for years.
The calculation of needed tirepressure for Bias Ply tires is what the tires organizations agreed a formula for in 1928, and for radial tires they chanched it about 1970 , different in America and Europe.
Nowadays the European formula for all kind of tires , is used in America since 2006 but only for P-tires ( in SL and XL//).
If you would have a Bias Ply and radial tire with same maximum load and pressure needed for that ( called reference-pressure in offical European Formula , I once got hold of) It would give lower pressure for the same load on tire, for bias ply then for radial ply.
The 26 psi given by capry racer as minimum for P tires , is in European system 21 psi/1.5 bar.