u/Aalju

This part is all rusty, and my mechanic that fixed my brakes (pads, discs, caliper) couldn't find a spare part of this or do research as to what this is. He said this is probably going to ruin my brakes again after a while if it's not changed. He wasn't proficient in english or my language finnish, and was quite old so his internet skills were probably lacking a bit. Could you help me out?