When even those were not enough he resorted to the old Egyptian division of Decans. Giving him 36 slots in which to store information. The technique remains the same throughout. Associational stepping stones are constructed between the thing to be remembered and something else which is already readily accessible to conscious memory. The art is in selecting, or creating, stepping stones which work well and are spaced out in an efficient manner.
Memory is a creative process.
Whether Bruno was a synesthete or not is a historical question mark. That for which no doubt exists is that he was evolving a synesthetic modus. And the lessons about memory are invaluable. Employing any categorical system which is ambiguous enough to accommodate many associative contexts, it is possible to train the mind to remember more. The possibility that synesthesia is exercisable, that is some sort of symbolically sensitive sensory muscle is alluring.