While you are on the site, in your browser find and click "Add to Home Screen" A CGN beaver app icon will then be created to your phone that is directly link to the site.
Note: This feature may not be available in some browsers.
They switched to the "N" prefix in 1969 and the model 28-3 came out in 1982.
The only way to find out for sure is to send the serial number to S&W and they will give you a letter of the date shipped and who it was originally shipped to for $50.
EDIT: From the serial number you PM'd me the shipped date would be 1978-80. That is as much as it can be narrowed down.
I purchased a model 29 last year from Epps. E-mailed S&W customer service. They replied two days later, my gun was manufactured in 1957. Didn't cost a dime, went strait to the horses mouth.