That is a nice looking Model 66
Only a Factory Letter will tell you for certain if it is Factory Original
However, based on a few of the tell tale signs, I would venture to say that is an aftermarket engraving job
The biggest tell is the Smith and Wesson logo. On most of the Factory engraved revolvers, they use the small logo on the left side and leave the side-plate open for embellishment
The side plate is also a great place for large embellishment
Like all rules of Smith and Wesson, there is always an exception or two. That is why you need to go the Factory letter route.
That is not to say that aftermarket engraved Smith and Wesson are not good pieces. Sometimes they are great values because the engravers are unknown or just starting out
I am extremely happy with this non-factory engraved 686
Value wise, your revolver would easily hit the $1500 mark on most of the auction sites.
The grips on your revolver now are the ones that were on it at the time of engraving. You can tell by the borders around the. That is another tell that this was not a factory job. Odds are the factory did not supply a set of non-medallion grips with a factory engraved revolver
Admittedly I do not live in Missouri, but I have never heard of an instance where a quality engraving job lowered the value of a Smith and Wesson revolver.