[bgar3]

While I can’t give an expert opinion on the image you question, I do believe it is meant to depict Chadwick. Like you, I have always heard it was Chadwick, it does look like him to me, and, importantly, the papers inscribed below his image were the one he worked for reporting on Base Ball.
As for other Chadwick images and dates , I can provide some information.
In general, dates for Chadwick images are estimates, very few exist that have dates the image was taken. (Ones I have seen with dates taken are those from Spalding guides, with Pearsall as the photographer. Pearsall is said to have taken Chadwick’s photo for many years on Chadwick’s birthday.)
That said, the image you use as a reference was taken no later than January 17, 1880 since it was used as the basis for a woodcut which appeared in the New York Clipper on that date. I also have a cabinet size photo of this image with an 1886 inscription. I have also attached an earlier and later image for your comparison. The oval image has been described as circa 1868 and I think that is fairly close. The white border cabinet is by Pearsall and I think is circa 1895 or so. (It was used in a 1924 Baseball magazine article.)
Someone who has a run of Spalding guides could check the for images and dates. Hope this helps.