The editorial lumbus–plumbum solution

Meaning

The reader’s six-line riddle asks for a six-letter name of a human body part, followed by the addition of p and a change from s to m. The supplied note proposes the Latin LUMBUS, “loin”: prefixing P gives PLUMBUS, and replacing the final S with M gives PLUMBUM, “lead”. This explains the letter operation without requiring the intermediate PLUMBUS to be a separately attested substance-name.

Those Latin solution-forms are modern explanatory material. They are not printed in the French poem and therefore belong in the English and editorial lookup, not among its historical French or Latin attestations. Even if the proposed solution is accepted, it does not establish that every philosophical subject in the treatise is ordinary lead. The later discussion of Saturn itself considers a lead interpretation only as a partial fit.