Just one comment about "factory engraved" guns. As mentioned above by "first-model", a factory engraved gun is one commissioned by S&W vs. a dealer or distributor. But not only could those non factory commissioned guns be done by the same engravers, but they could easily be done at the same bench.
The factories had engravers they contracted with, but in almost every instance none of those "in house engravers" actually worked in the various factories. They worked out of their own shop and the guns were either picked up or delivered to the engraver's shop where the work was done.