Condor, don't feel bad. It's a very complicated subject. Pressure does not relate directly to either velocity or shot charge. But velocity and shot charge are the most important components of recoil, and reducing recoil is a very good thing for old guns to avoid cracking old and maybe oil-soaked stocks.
It's my understanding that the major American ammo makers don't put pressure on shotshell boxes because they sometimes change from one powder to another. They strive for 2 things: Constant velocity, and pressure within the 11,500 psi SAAMI limit (for 12's). They don't really care about pressure because they assume you're shooting a gun that meets SAAMI standards. European shotshells, on the other hand, have to conform to CIP standards--which means that if they're for standard proof guns, the pressure ceiling is about 800 psi lower vs SAAMI. In this country, RST and Polywad offer low pressure loads, and Kent (Gamebore) and B&P shells that meet European standards are also available.
With older guns, you want to hold down both pressure and recoil. One reason American factory loads--especially some of the very fast hunting loads--are bad for vintage guns is, even if there's not a pressure issue, the velocity results in a lot of recoil. You don't need 14-1500 fps in a lead shot load to kill pheasants anyhow. 1200 fps or so has worked quite well for a very long time, and the last few I've cleaned haven't been wearing Kevlar.
