I wouldn't trust any S&W to be "factory" engraved without a factory letter saying it is so. I have found that most early S&W .38 and .32 top breaks that are engraved were done so at the request of the wholesaler. (Like M.W.Robinson of New York.) These will not "letter" as being an engraved gun, but it doesn't mean that they aren't valuable.
I have an engraved Model 1 1/2 that letters as having been sent to M.W. Robinson, a New York wholesaler, in 1870. It was just a "Plain Jane" when shipped, but has since been accredited to Nimschke for the engraving.
It still is a valuable piece.