Welcome. I have 38 M&P, #81,XXX that shipped in June 1906. Those original stocks are very nice and are valuable in that condition so don't separate them from the gun. As for the gun, I think $400 is generous for the brown finish, but looks like you have a very serviceable revolver that has not been touched up or refinished ever.
About the only way externally to tell whether you have a 1st Change or 2nd Change is looking at the extractor star as Guy says. The 1st Change extractor arms had rounded ends and the 2nd Change had square ends. Push the extractor rod rearward and look at the rear of the cylinder for a small alignment pin. The presence of the pin indicates 2nd Model.
I have a few guns in the gray area, #73,251 to #146,899 and all I have lack the 2nd Change extractor features. I have 81,XXX, 100,XXX, 106,XXX, 113,XXX, 129,XXX without any 2nd Change features, which indicates the 2nd Change did not happen until late in that serial number range.