At the present time a letter costs $50.00. For the same service Colt charges double that.
For this fee they will go back through the records (which are not computerized, and the revolvers are not listed in order). The letter will give you a short history of the model, followed with specifics about your gun. Usually this includes a full description, the date it was shipped, and to what distributor, dealer or whoever.
If the letter (big "if") confirms that the piece was engraved at the factory and the ivory stocks are original the value can go from several hundred to a thousand or more.
There is another possibility. At that time Smith & Wesson would sell revolvers to certain large distributors "in the white," with no finish. The distributor would then have them engraved, add fancy grips, and either have the gun plated, or send the revolver back to the company to have the finishing done. If the letter says that it was sold "in the white," this is what likely happened. These examples are worth less then a factory engraved gun, but more then one that was done by an unknown aftermarket engraver.
In any case it's unlikely anyone will pay big bucks for a gun that doesn't come with factory confirmation, but on the other hand you can add the cost of the letter to the price of the piece, no matter how things come out.