They dont seem like threads, seem concentric. When i first saw this i thought of fidgeting while bored. Kind of fuzzy. Wouldnt be surprised if was on a fishing pole. Like a piece that would fall off a cork handle. We used to fish alot as a kid and i was bored out of my skull in the 70s. The original fidget spinner i guess......you knew that was coming.
I also have been a pipe fitter for 40 + years and I have seen pipes rust thru in a few years depending on the environment. But being the size of a quarter I would say it is closer to being 3/8 npt