Thought the following exchange with S&W customer support might be educational to others.

Questions sent to S&W:
Dear Sir or Madam,

I have a question regarding serial numbers. Let us say that a particular gun has the following problem with its serial numbering: Somebody has filed on the yoke and frame under yoke (ostensibly to fix a yoke clearance problem) such that the last digit of the serial number there is rubbed off. The serial number on the butt of the frame is fine and unaffected. What sort of problems could this create for the owner?

If a gun with this serial numbering issue were sent to S&W for repair, would you work on it? Could you even re-stamp the serial number on the frame under the yoke?

Answers from S&W CS:
I would check with BATF on the first questions.

No, we would not re-stamp the serial number.

If the gun were sent in we would probably work on it because the serial number is wholly visible on the butt.

Situations as you describe are a case by case basis.