It's a "tough call" to make on the ID of your brass spike-shaped find, for the following reasons.
1- It is shaped like a late-1800s-to-early-1900s brass/copper lightning rod top, which (like your find) screwed onto an iron rod... but yours looks a bit smaller than other lightning rod tops I've ever seen. (Check the photo below to see the typical size.)
2- Your find, having internal threading inside its base, is made to screw onto the end of a METAL shaft, just like the antique lightning rod tops I mentioned above. Most hand-carried flagpoles were (and still are) made of wood -- therefore, their "finial" (ornamental top) did not have a INTERNALLY-threaded cap at the finial's base.
So, taking all of the facts listed above into account, I have to lean toward thinking your find is not a flagpole top... but instead, is either an unusually short top/finial for a lightning rod, or is some other version of decorative finial. Here's the photo of a late-1800s/early-1900s lightning rod, with the copperbrass spike-like finial screwed onto the upper end of an iron rod.