Hi Jinxy, Looking at the manual you need to loosen both the locking nut (in the curved slot) and the nut holding the gears C & B . The locking nut should allow the whole pack to swing down and the Tee bolt holding B & C should then be able to slide to accommodate the different gear sizes.
The photo looks a lot like ones I had as a boy from a book called The Boy Mechanic. First published in 1913, the one I have is the New and Revised edition published in 1960. It was published by Popular Mechanics.