I am confused about lots of things, but with what we had to work with, I think I made a reasonable guess. You called the SS a Model 1891, which has the recoil shield on the frame as a 38 3rd Model Single Action revolver, which was used to manufacture the 1st series of single shot pistols. Now that you state there are grooves in the frame instead of recoil shield, it would be properly named Third Model Single Shot. The original barrel would not have had the Model of 1891 stamped, but would have had the walnut medallion stocks, so that is a good thing.

I did notice that your serial number is above the 7787, which could identify it as a 1st or 3rd Model, but without pictures, you could have had either. Post a photo or two. Your 3rd Model would have most likely been shipped in 1916.

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Gary
SWCA 2515