The top sideplate screw was deleted in 1955 at approximately SN 11,000. That gun is probably 1957, right in the transition period for model marking. Does the SN on the box match the gun? If it does, what does the box say the model is?

The 1955 in the box is no more than the date the language of the message printed in the box was adopted. They must have been satisfied, because the same wording, and date, was used in all S&W boxes until they discontinued the message sometime in the 1980s.