Where to start? First, welcome to the Forum. The revolver is certainly a Smith & Wesson, but not an Old Old Model 3 since that serial number range was 1 - 20,000 and another Russian contract that ran in the 44 American serial number range of 6,0000 - 32,800. Your serial number is 66,916 so falls into the Model 3 Russian Second Model, or Old Model 3 Russian.
In the 1870s, the Russian government purchased over 100,000 revolvers in three different model lines. Your revolver would have left the factory with a 7" barrel and a long ejector housing as you show. I believe the front sight was an attempt to add a new sight for more accurate shooting. I would try to remove the fixture if it is not soldered on and see if there is a narrow slot in the top. If so, a front sight could be fashioned and installed to look original. There was also likely a hook on the bottom of the revolver's trigger guard that was removed. The "Russian Model" stamp means that revolver was sold as a commercial gun and was not part of the Russian contract.
I believe that what sits on the top of the "E" in the oval is a crown, not a bomb. I have never seen a Belgium stamp with just the E in the oval, as all had an ELG, meaning "Epreuve de Liege" or Proof of Liege. There was a stamp used on French guns that was a crown over an "E", but without the oval?? Also, Epreuve is a French word meaning proof, so maybe French??
Here is what the revolver would have looked like when it left the factory.
