OP, I see in one of your other posts that you asked about getting a Letter on your engraved Model 60's.
It is a toss-up, especially if you are contemplating selling the guns. A Factory Letter of Authentication can be helpful IF...IF.. the engraving was done at the factory, prior to shipment to first owner.
Otherwise (aftermarket engraver) the Letter will only state that the grips and engraving are aftermarket and you are still in the dark about who did the engraving.
The below posted Letter is in regards to the Ben Shostle engraved, with stag grips that I posted above. The Letter did not make any additional "value" when I sold the revolver, but then again...the deal I've found with engraved firearms is that even if you are priced fairly...it still takes that one buyer that really appreciates engraved guns. Other shooters and even collectors just see a tremendously overpriced Model 60, so you are really looking for a fine art connoisseur who also appreciates fine firearms.