Question: in 1908 were they proofed for smokeless or black powder like the 1880’s Safety Hammerless?
An excellent question, and I cannot give you an authoritative answer.
With Colts, it is clear. By 1900 Colt felt that the steel they were using, as well as the heat treating processes they were using were good enough so that Colt warrantied the Single Action Army for Smokeless powder in 1900.
I have searched high and low but I can fine no authoritative source on when S&W felt it was safe to fire Smokeless powder in their revolvers.
I have a reprint of a 1900 S&W catalog, and S&W is warning against shooting Smokeless powder in their revolvers in that catalog.
Smith and Wesson developed the 38 Special cartridge in 1899, and at the time it was loaded with Black Powder.
Here is a page from the 1900 S&W catalog regarding the new 38 Special cartridge.
The 1905/1906 catalog is ambiguous. regarding the use of cartridges loaded with Smokeless powder. In it S&W cautions against the use of reloaded ammunition made with Smokeless powder, citing their view that the quality of the powder ammunition loaded with Smokeless powder may be questionable.
Quote:"Cartridges in which Smokeless powder is used are made by leading manufacturers, and they possess valuable qualities not found in Black Powder ammunition. They do excellent work in our revolvers, and
while we do not guarantee our arms when Smokeless powder is used, and strongly advise against reloaded Smokeless ammunition, we have no desire to detract from its merits or discourage its use when properly handled"
Note: the Italics are mine.
So S&W was hedging their bets, not wanting to take responsibility if somebody blew up one of their revolvers with ammunition loaded with Smokeless powder.
Practically speaking, I don't believe Colt had access to better steel than S&W did about 25 miles up the Connecticut River, but S&W seemed to be hesitant to guarantee their revolvers for Smokeless powder.
We know that the 44 Special cartridge was developed by S&W in 1907, originally loaded with Black Powder. We know that when the Triple Lock was introduced in 1908, it was the first cartridge chambered for the 44 Special cartridge, and by that time I have found no mention that the Triple Lock should not be fired with Smokeless ammunition, and the same for the 38 Military and Police (which eventually became the Model 10).
Sorry, that is the best I can do. I suspect those bicycle revolvers of mine from 1907 and 1908 would be OK to shoot with Smokeless Powder, but I am not about to do so.