Take it for what it's worth, but the Wikipedia page for .38 Special does make the statement that ammunition chamber pressures and velocities were reduced by the manufacturers in 1972. And I believe that it is true. This is what is said: "It is important to recognize that SAAMI changed the specifications for the 38 Special in 1972. Prior to that time the standard .38 Special was very close to today's "+P" cartridges."

I have understood that the principal reason for this was that different laboratory methods for pressure testing (the old copper crusher vs the new Piezoelectric pressure gauges) were incorporated about that time, and provided a far more precise means for measuring chamber pressure vs. time. The old copper crusher method could not come close to matching the precision of the piezo gauge, and in fact was incapable of measuring peak pressure at all. With this tool at hand, the whole concept of maximum chamber pressure was re-visited, resulting in lowering chamber pressure maximum limits across the board.

If anyone from any of the ammunition companies or SAAMI is reading this, perhaps they could supply a more complete explanation as to what happened 40 years ago.