What’s is this?
do I need it?
Where can I get one?

Fell off the inside end of the lower bolt (slider?) on the front brake. Left half of itself on the bolt, other end tore off? Hard to search for online when you don’t know what it’s called.

Ta.

 

That is part of the boot that keep crud out the slider pin. You need it. Without it that pin can seize, and the caliper will not "float" any longer.

https://www.atkinsrotary.com/Brake-S...41-49-250.html

 

Go to a local brake specialist. It's a fairly generic part and they will have them.

 

Got it. Parts on the way. Thanks!

 

Make sure to use some brake or bearing grease on the slider pin.

 

Will do. Have grease left over from rear caliper rebuilds.