The original M&P revolver was the model 1899. The short lived Model 1902 (no change) added a lug under the barrel to support the front of the ejector rod (and make it easier to tell from a Colt). This variation was made only approximately 1902-1903, and only in round butt form. The 1902 first change reinforced the rear of the barrel, adding a larger diameter collar where it met the frame. This remained through all future M&P models.
Here are two examples of 6.5" no change target models, the top one in .32-20 #6421 and the lower in .38 Special #31771.
Bob
Here are two examples of 6.5" no change target models, the top one in .32-20 #6421 and the lower in .38 Special #31771.

Bob