It is possible of course, that various Manufacturers were using differing Charges in the Black Powder .38 S & W Special Cartridges...either intentionally or in error or from some problems with their Mechanized Loading Machines' settings.

18 Grains of Black Powder is what the slightly shorter and less powerful for any given Bullet weight .38 Colt ( aka U S Service Cartridge ) used, and, for .38 Special to duplicate the .38 Colt Loading, would be entirely self defeating or irrational also, given that the intention and advantage was that .38 S & W Special was a more powerful Cartridge for charge and Bullet Weight both.

18 Grains of BP in the .38 S & W Special Baloon Head Cartridge of the day, would not allow good compression once the standard 158 Grain RNL Bullet was seated to correct depth...

So, one way or another, it seems to me, that there is something odd with that 18 Grain Loading being referenced as if it were the correct or Standard Loading, even if it was noted by an expert, to have occurred in a period UMC .38 S & W Special Cartridge.

I believe such an occurance would have to have been an anomoly.