And given it's an M&P, it was shipped late nineteen teens---early twenties---and the star on the butt along side the serial number is a factory service mark-----it's been back to the factory for one thing or another---and there may be a date code on the left side of the grip frame. If it was in August, for example, it would be 8, followed by two digits indicating the year. And considering the fact there are "different strokes for different folks", it could look like so: 8XX, or 8 XX, or 8-XX, or 8/XX ----or nothing, because it was too early in the morning, or too late in the afternoon, or because he just didn't feel like it.
AND----IF the S&W Historical Foundation has Service data for whatever year it was, they may be able to tell you what the service was---and how much it cost---and the name of whoever sent it in---and where it came from---and who knows what else! You won't know until you ask, and it doesn't cost anything for the search---and chump change for copies of what they do find. The foundation has a section here on the forum, so you can contact them, and see what's what.
Ralph Tremaine