The V serial was used only during WWII, but it came about in an interesting way. S&W was already making M&P revolvers under contract with the British and some other countries when the serial numbers approached one million. Now S&W's numbering machine could handle only six digits, so they were considering using a letter prefix which could be stamped on all the guns during initial production before the rest of the serial number was put on. They planned to use the obvious "A", but even though the US was not yet in the war (Nov. 1941), the slogan "V for Victory" was well known, especially in Britain. So the letter "V" was chosen instead, and most collectors call any M&P with a "V" prefix serial number a "Victory Model".
But S&W didn't, and that causes some confusion. The factory called only those revolvers made in .38 Special under U.S. contract "Victory Models". Revolvers made for the British or for Lend Lease in .38/200 (.38 S&W) were called the ".38/200 Model", not the Victory Model.
So the normal M&P numbering went to 999,999, then to V1. In Dec. 1944, the new hammer block safety was added, and the letter S was included in the serial number; its location varied, but there is no special significance to the exact location.
Jim