A Little What is It?

[pawngal]

Just spitballin here but I'm guessing the black end is like a little rubber cup-shaped tip that you push onto the crown? And it stops you from over-winding by "slipping" when the watch is fully wound?

You are correct

 

[BC38]

You are correct

Interesting. I'd have never even imagined there was such a thing.
But I haven't worn a watch that needed winding in probably 45 years or more. I've been wearing a watch every day since I was five or so, but started wearing battery operated watches as a teen when they became common enough to be inexpensive.