It's chambered for .38 Special, aka .38 S&W Special. Cartridges are named by the company that designed it and many companies included their name in the designation.
.38 S&W and .38 S&W Special are two different cartridges and they are not interchangeable.
I have't seen that marking on any of my S&W's (with the /). But I'am pretty sure it means that the gun is chambered for the regular 38 Special cartridge. FYI the current Remington ONLY brand of the old 38 S&W cartridge fits and fires fine in a 38 Special revolver. Have a great day.