Not so run of the mill.
What you have is an "S" series post war M&P, they were manufactured between 1945 and 1948. S999xxx was the last so yours is a late one. Someone will stop by and give you a more accurate date.
There's a lot of interest in them here on the forum, more from the variations as they evolved into what became the Model 10.
I have 2, they're great shooters.
Love to see some pictures.