I think what the jist of it is its a tool for possibly checking tolerances in the making of the cartridge Muffett as to what end is only being surmised but I stand by what I think it is, also has a scale on the spindle similar to a Redding BR powder throwers so I gather the measurement could be a +- for it to be serviceable.
But as you say if the stenciled writing on the tool be deciphered and put on the forum then this may enlighten all of us.