Shocks work by trying to force oil via a piston/(vane in the case of a Model A) thru a small hole. The smaller the hole the stiffer they are. It is like trying to force sand thru an hour glass.
https://www.koni.com/en-US/Cars/Tech...ng-Principles/
Gas pressure is added to stop the oil foaming. Foaming causes the shocks to fade as gas can compress. Shock fading is a problem in our outback on poor roads at reasonable speed in hot weather. They can also heat up and burst as they cannot dump heat fast enough.
It may be that gas pressure shocks are made harder? For a Model A with lots of friction between the leaves you won't need hard shocks.