Oh ok so 1905 is the model and this is a first change of that model which puts it aprox at 1906? Was this before they started designating frames like K-frame and J-frame?
These explanations are too comprehensive to reiterate them in this thread. You need to read the Standard Catalog of S&W, 5th Edition to fully understand the chronology of development of S&W handguns.
At the time your hand ejector was made, there were top break revolvers and hand ejectors made at the same time. The hand ejectors were in three frames: the M frame, the I frame and the K frame. The M frame was a tiny gun that chambered .22 Long cartridges. The small I frame chambered .32 S&W Long and the mid-sized K frame chambered .38 Special. Your .38 M&P was first made in 1899, majorly revised in 1902 and again, majorly, in 1905. There were changes made after each major revision but before the next major revision. Collectors track these changes because some affect the value of the gun more than others. Again, there is more detail in the Standard Catalog that will help you understand the collector terminology.