One of my older smiths is marked, "38 S&W special US Service Ctg". I know Ctg means cartridge, what about the rest?

The message is both cartridges work. That's the end of what I know for sure and certain. I THINK I remember the U.S. Service Ctg is another name for the .38 Long Colt---and S&W was supposedly disinclined to put the word Colt on their guns.

Ralph Tremaine

And I just looked to be sure, but you seem to have missed the little ampersand goody (&) in front of U.S. Service---and the little 's after Ctg at the end of it----probably would have simplified your life if you'd seen those. Be that as it may, the earth is still spinning on its axis---as in "Don't sweat the small stuff!"---unless there's money riding on it!

The U. S. Service Ctg is the .38 Long Colt. It was the official U. S. military revolver cartridge until the .45 ACP. Its case is slightly shorter and it's less powerful than the .38 Special but is otherwise dimensionally identical. Like shooting a .22 Short in a .22 LR chamber.

Ralph was right, I did miss the letter "S" at the end of Ctg, now it makes sense. Thanks to Ralph and DWALT.