That's a model of 1905, First Change. It probably shipped in 1906 or 1907. The first-change guns were the first ones in the M&P line to have the familiar rebound slide mechanism instead of the pivoting arm driven by a second flat spring mounted in the grip frame.

The "U.S Service Ctg" does not mean .38 S&W; it refers to the .38 Colt round that the government adopted a few years earlier and found too light for reliable military purposes in some Philippine encounters. The .38 Special is in essence a longer version of the same cartridge. Within a few years the .38 Special became standard for both companies, and the Colt round became history.

A good welder should be able to put the hammer back together for you. Otherwise, check out oldwestgunsmith.com, Jack First in South Dakota, or Poppert's gun parts in your search for a replacement hammer. I speculate that a no-change hammer will be slightly different from later 1905 hammers because of the interplay between hammer and rebound slide in the later changes. But I don't know if that will make the later hammers unusable in your gun. Maybe one could be modified to fit.

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David Wilson