From SAAMI:
For many years gun chamber pressure units had been commonly referred to as “pounds per square inch”, which was not technically correct. The older method of pressure measurement involves a piston through the side of the chamber compressing a lead or copper cylinder in which the measurement of the degree of compression is indicative of the maximum relative pressure generated. With the advent of the electronic transducer, it became necessary to indicate by some means the method and equipment used to determine the pressure values given. This is important, since the pressure values determined by one method cannot be mathematically converted to values for another, despite claims to the contrary. Likewise, the limiting pressure values for the different systems are not interchangeable.
SAAMI created the designations of “Lead Units of Pressure” (abbreviated LUP) and “Copper Units of Pressure” (abbreviated CUP) to clearly indicate the system used in determining pressure results and/or limits. These designations apply only to values with the particular crushers, test gages and methods as outlined in SAAMI technical procedures. The terms LUP and CUP represented a change in name only. The pressure testing equipment, techniques and the numbers themselves are essentially the same as those associated with pressure units expressed as so many “pounds per square inch” prior to the advent of the piezoelectric transducer method. The term “psi” (pounds per square inch) is now reserved for electronic (piezoelectric) methods of measuring pressure, which is the predominant system in use today.