Thanks! I knew this was the right place to askThey are for calculating shim thickness when aligning connections. Such as a motor to a pump. Used similar to feeler gauges, stack them together and fit them under the base. Held/manouvered by the apex of the triangle. The slot is to get around the mounting stud/s Ruler is to give you an idea of the size of the required shim, and also where the required stud holes/slots will need to be drilled.
Cheers Phil