The presence of the original (fixed) rear sight is not the least bit unusual. The Lyman (front) sight was available on special order----with or without the Lyman rear sight (The Lyman Ivory Slide Sight). The patent date on yours suggests it's not the original sight for your gun-----but??!! If (BIG IF), the rear sight on your gun is factory (I doubt it.), then Jinks will be able to tell you about it. (I think it's not factory simply because I've never seen one like it-----------which proves, beyond a shadow of a doubt, absolutely nothing!) Even if your rear sight is/was a part manufactured for the aftermarket, rather than a one-off by a gunsmith, then there's a better than even chance Jinks will know about it.

There is a book (Old Gunsights---A Collector's Guide, 1850-1965; Krause Publications, 800 258 0929 (for orders or a free catalog); 715 445 2214 otherwise) I have for reference. I have not looked at each and every page, but I dug it out and went through the S&W section again. There are no pictures for this, but mention is made of an "Elevating Rear Sight" for the 38 Double Action and other "Jointed Frame Revolvers". Your latest picture says (to me) the sight is not adjustable for elevation, but is it-----an elevation screw underneath----either in plain sight or somewhat hidden toward the front-----ahead of the cylinder retaining lug?

Finding out about your sight may not be all that difficult----if it's an aftermarket item rather than custom, made to order item. Take some good pictures (top/bottom/both sides) and send them off to David Carroll and Ed Cornett. David has seen a ton of guns in his time, and Ed is a walking encyclopedia.

Ralph Tremaine