These C spanners are cut from 6mm thick steel, deburred and then powder coated. Made to fit the exhaust nuts on OHV and OHC Norton singles from 1932 onwards. 
 
They allow you to tighten up or undo the nuts with little or no danger of damaging fins, despite how tight they can sometimes be. 
 
These spanners are very solid but still small enough to fit inside a standard Norton tool box to you can carry it with you on the bike.