I agree with Dean, that it is most probably aftermarket engraving done in one of the large New York distributer shops, or commissioned by them. It is not what I would call cheap pawn shop style engraving. The New York distributers had some excellent engravers working for them. The only real way to tell for sure if it is factory engraving is to have it lettered. I have several engraved guns from the period and two in particular are indicative of the difficulty of determining the origin. They look identical to me and one letters as factory engraving and the other is listed as being shipped to MW Robinson as a plain nickel gun. That's a nice old top break there in any event.
Mark

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Mark Connot