I don't know of a version of either the hammer song or the ballad dated reasonably to earlier than 1905.

Of course, there is plenty of testimony of earlier singing. Some of this testimony has the "John Henry" hammer song being sung as early as 1871 or so, the time of the building of Big Bend Tunnel. I see no reason to accept this - there is no consistent testimony placing John Henry at Big Bend or having him sung about there. I think the people who gave the testimony probably heard some hammer song at Big Bend, then later heard a John Henry verse or two sung to the same tune and subsequently conflated the earlier and later versions in their minds.

Chappell lists a few claims of "John Henry" songs being sung before 1888 but the big explosion of remembrances starts then and is dense for the years immediately following.

One man said that he moved to Georgia in 1888 and found everyone there singing "John Henry." This fits very well with 1887 as the year of John Henry's death and with Alabama as the place. Men from Mississippi worked on the Columbus & Western job and I'm sure men from Georgia did, too. After all, the Columbus & Western was owned by the Central Railroad and Banking Company of Georgia. "Columbus" is Columbus, Georgia.

I suspect that "John Henry" was already being sung in West Virginia in 1888. There is testimony that laborers who worked on the C & W went to West Virginia to work on the Elkhorn Tunnel, which was completed in 1888, as I recall. When the ballad hit the Big Bend area, people there remembered Big Bend's prominent steel driver, John Henry Martin.   They probably identified him with the legendary figure and localized the ballad to Big Bend.