I made an error in the chambering in my post above. The original guns were made for a cartridge having a heel type bullet, which had the same outside diameter as the case, like a .22 LR. Those chambers were bored straight through and will accept .38 Special and even .357 Magnum.
When Colt changed over to .38 Special about 1903, they put shoulders in the chambers. Those will accept .38 Special, as they were intended to, but not .357. That went along with the change in barrel diameters. I can find nothing to indicate that any change in the metal used was made when the change to .38 Special was made. No change was made in the barrel marking.
To summarize, with guns in good condition, .38 Special should be safe enough with the standard loading; +P, +P+, and .357 Magnum should never be used. As to the black/smokeless powder issue, Frankford loaded the .38 Long Colt cartridge with black powder into 1900, when they changed to smokeless; I have seen no indication that the latter was not intended for normal use. Commercial ammunition was loaded with smokeless prior to that date. The cylinder material and diameter is the same as those of the succeeding Colt Army Special, which was made in the smokeless powder era.
Jim