Howdy, again, Pard!
The only reference to Doc's "nephew" was what was in Guns of the Gunfighters. Even though E.J. Williams and I surveyed many hundreds of the Remington derringers (the spell checker just added in the second "r") in preparation for the 1995 article in Man-At-Arms, and later as co-authors of the book, it was certainly impossible to have seen more than a few percent of the total production over the 69 years of production by the various incarnations of "Remington". One thing we did find was that there was overlap between the various "models". The reason is that what we call "serial numbers" were really not. Rather, they were batch numbers for the purpose of matching barrels to frames, and for inventory control. One of the minor, but notable situations in this regard, was the early "Remington-UMC" guns (Fourth Models by our terminology) with three-digit numbers. For the most part, these had thumbpieces and hammer knurling like those seen on the Remington Arms Co. guns. Also you can find a number of Fourth Model with the L prefix with date codes out of order of the serial numbers. How could this be? Likely because the gun barrels and frames were matched to each other, but before final assembly, were simply tossed into a parts bin, and then given final finishing and assembly in a "last-in, first-out" order. The date codes were then stamped on the lumps under the barrels. Similarly, Second (two-line address on top of the barrel) Model guns might have early 3rd Model parts installed. (Why bother re-stamping the new company name on finished barrels?)
Bottom line, I bow to your documentation of the authenticity of Doc's derringer (there...it did it again!), and apologize for casting any doubt on it. Too bad these guns didn't come with speakers and flash drives so they could talk to us!
I've owned several different original guns that I wish could tell me their stories directly. No matter how hard I listen, they don't speak to me...at least in audible words. Sometimes, though, there is enough documentation here and there...
Happy Thanksgiving!