The .38 S&W was first introduced in 1876 in the spur triggered S&W First Model. It was originally loaded with with a 146 gr. lead round nose bullet at 730 FPS.
I could not find the exact black powder charge used, but here's what gun writer Mike Venturino had to say about black powder metallic pistol cartridges...
"Fill The Tank
An oft-asked question is, "How much black powder should I load for cartridge 'X'?" It doesn't matter. Just fill up the case so the bullet's base compresses the powder slightly--say about 1/16". The brand of case and seating depth of the bullet will determine how much powder fits. Don't worry--you can't get in too much black powder to harm a handgun in good condition. If it's not in good condition you shouldn't be firing it anyway?"