The 38/40 was originally a black powder cartridge. A .38 caliber bullet in front of 40 grains of black powder is what the name intends.
A very popular caliber in it's day. Similar to a 44/40 (.44 cal bullet in front of 40 grains of BP).
From what I have heard, the 38/40 was a very popular deer cartridge in rifle, before the 30/30 came on the scene.
R
Hank