The original poster did not write that his buying anything. He wrote that in 2013 a dealer used a K prefix number on paper work for his Model 28. I think we all know that can not be correct.

My habit is to verify the serial number that FFLs write on the forms that they fill out in front of me. I have caught them making innocent mistakes, the most common being using a S&W revolver's assembly number.

One risk in not correcting them is that if they also sent the wrong number to the police they could unknowingly sell you a stolen gun. You could later loose that gun while transferring it through an FFL for any reason.

All the N frames that I bought new in the 1970s and 1980s had a duplicate serial number stamp in the yoke cut out along with the model number. Generally, the assembly number was moved from that location to the left side of the gripframe when S&W started stamping model numbers there. By the early 1990s the Feds had started requiring all new guns have their serial number in two places on their frame or receiver. By the way, some post 1990 S&W revolvers have the serial number on the under side of the top strap or on the side of the frame below the cylinder window and some did not use the butt for one of the two serial number locations.