Alk8944 is right on the money. The image below was taken from the 1972 Guns Annual. Note the test barrel lengths and remember they where likely unvented pressure test barrels and not real revolvers or semi auto's. No real difference to today's ammo when one subtracts the expected velocity gain from longer barrel and no cylinder gap except the term +P wasn't in use. Higher performance ammo was called Police loads or Hi-speed which was the equivalent to today's +P.