The authenticity of a horn with a spring loaded brass measure spout would all depend on the time period you are planning on recreating and the persona you are trying to recreate.
Bear in mind that the features in question are generally found on horns owned by the upper class or social elite. They were expensive, hard to make and rare. If you are building a persona based on something like a HB trapper, woodsman or someone of that nature, than a brass tipped horn would probably not be something for you.
The introduction of spring loaded spouts on horns are hard to date. The practice grew overseas and migrated to the US later; but the first examples I have seen were really no earlier than about 1820. Granted, it is always important to remember that when trying to date anything from original examples, you should consider the common museum practice of taking an additional 10 years or so from the dated item.
Unless the specimen in question is a known and documented example of a "first type" or is a prototype taken from the inventors own desk; most items of this nature found in museums had the origins of their features evolve some years back from the artifact you are looking at. If you see a powder horn with some kind of brass tip in a museum and it says "Brass tipped powder horn, maker unknown. Circa 1842) You can bet there was one like it, somewhere, at least 10 years earlier.
Experimentation with spring release nozzles for priming horns can be found slightly earlier than 1800, but not much and not in great numbers. Many of the first horns with brass measures attached to the nozzles were in fact a bit of a Frankenstein arrangement in which the maker had cut the top off of a very early brass powder flask and attached it to the end of a horn.
Keeping true to history, it would be hard to justify a fancy spring loaded, brass tipped powder horn in the hands of the common trapper or woodsman. Most men of that ilk would have little use for such an item. Their skill, built up over years and years of nearly daily firearm use would have honed their skills with a horn far beyond the need of a fancy measure. If such a persona is your goal than a standard horn with a regular wooden stopper, complete with bite marks, would be the more accurate accoutrement IMHO.
If your impression is to be that of a city gentlemen, say from around the 1830's, than the inclusion of such an item would be a great addition and would add a subtle bit of class to the overall look of your persona.
:m2c:
