S&W M&P (pre-Model 10). 2" round butt, s/n 430xxx, no letter prefix.
It's a Model of 1905 4th Change, built sometime between 1915 and 1942. However, the 430xxx s/n is in the Depression-era timeframe when S&W was assembling guns from stockpiled parts with no regards to maintaining the serial number order. The only
way you'll get a build date for this gun is to letter it.
BTW does the 2" barrel appear to be original? Original prewar M&P snubbies seem less common than their postwar cousins, but many, many of them have had their barrels cut.