The first photographs make the finish appear dark, and I initially suspected a refinish. But, the fifth photograph makes me suspect otherwise, for the following reasons:

1) The grips look nearly new. If a firearm is refinished, it is usually due to use & wear, and the grips should also exhibit more wear to reflect previous significant use;

2) It is unusual for refinished firearms to still display vivid case colouring, as is the case with the trigger and hammer. Usually the case colouring would have dulled from use and would appear as such or would now be blued due to refinishing.

3) Likewise, if reblued, it would be unlikely that the knob on the ejector rod would not also have been blued, especially since the contrast between its pristine state and the extractor rod would not nearly be as distinct as the knob and the extractor rod would both have dulled from use;

4) The Smith & Wesson logo when viewed at an angle, as is the case in the fifth photograph, appears crisp and is not dulled. This often becomes much less crisp when reblued;

5) All edges appear sharp. Polishing prior to refinishing often leads to rounded corners and edges.

If this is refinished, which I think is unlikely, I would think that it would have occurred at Smith & Wesson. I would remove the grips to see if a month and year is stamped within, indicating the date of refinish.

And, if this IS a refinish, other than factory work, I'll eat my hat!

It sounds like the OP might have purchased this at a reasonable price. I have one, in not as nearly as nice condition, that I picked up for a song a few years ago. Given that I had so little into it, I reasoned that a factory letter was warranted, simply because even with a factory letter, I still had very little into it, and sometimes a letter can produce fascinating results.