As stated above the .38 Special was originally designed as a black powder round (the longer case would hold more BP than the .38 Long Colt case), but smokeless loadings were very quickly adopted. Factory .38 Special BP loadings were still available until the mid-1930s. I have no idea why anyone would want to buy them that late.

Black powder is not a good choice if smokeless powder is available. It gunks up the gun in short order. Your M1899 will safely handle light loads of smokeless powder. Wadcutter target loads would be very good for use in it.