Is it the method of marking the serial number being a series of dots versus whole digit stamps or the content of the marking itself that you are questioning?
If the series of dots, it's a dot matrix serial number engraving, used probably late 80's into the 90's before S&W went to laser engraving serial numbers. It's just like the idea behind the old track feed paper computer printers that made the characters from a series of dots, except it punched dots into the metal frame versus printing dots of ink.