Mr.PowerGuitar Strap for Guitar Bass with Shoulder Pad, Black, 29 - 52 in

What is the string in the package for?

Will this strap fit acoustic guitar strap button with jack? These are a bit larger than a standard strap button.

Does the strap connect to the studs well? It seems like it would slip easily...

Is the lock clip plastic or metal? I'm a little concerned about it breaking.