This tool can be bolted to the wheel studs on Early Post War cars and Silver Clouds to verify and/or adjust the perpendicularity of the brake shoe to the wheel flange using the extended pointer arm as a reference.  The triangle plate is zinc plated steel and the pointer is stainless.  The procedure is documented in the Factory Service Manual under Clouds, Section G, page 24:

http://rrtechnical.info/sc/sc1/wshop/7.pdf