Your search for "brake backing plate mounting hardware" revealed the following:
What Trailer Brake Assembly Do I Need?
Trailer brake assemblies are not selected by diameter alone. Two backing plates can look nearly identical and still be incompatible because of shoe width, axle rating, mounting flange, left/right orientation, adjustment style, drum size, or brake type. Match the system, not the appearance. Start with the brake dimensions and axle...
How Does a Trailer Breakaway System Work? Switch, Battery & Wiring Explained
Your normal trailer brakes depend on the tow vehicle and brake controller. A breakaway system exists for the moment those are no longer connected. If the trailer separates from the tow vehicle, a cable pulls the pin from the breakaway switch. That switch connects the trailer's onboard emergency battery to...