I think a factory letter would say that this is "after market" (i.e. distributor had engraved. Probably M.W. Robinson) New York type engraving done in the style of Nimschke.
Here are a couple of pictures of my 1 1/2 that one of the top S&W dealer/collectors has attributed to Nimschke. His comments were that the frame shows the hand of the master, but the cylinder and barrel looks to be done by someone else in Nimschke's shop. This was not uncommon.
(Notice the fine border work on the frame that is a hallmark of Nimschke.}
