

What is this Jaguar piece called?
You guys helped me learn a lot about this replacement bridge on my dad’s old 1967 Jaguar. Now I’m curious what the piece is called that is usually found where the screw is on the bridge between the D and G strings?
In the first photo, my dad’s guitar has a flat head screw where this piece is but I see a lot of Jaguars with a little button. I’m thinking it locks the tremolo but I’m not sure.
Any chance the screw is original in my dad’s guitar or was it likely replaced and if so, why would he have replaced it with a screw?