In 1924, Hupfeld launched metal pneumatics as a major innovation in the design of the pneumatic system. A key advantage of this system was that each metal pneumatic unit for each individual note could very easily be removed, checked, cleaned and reinserted, or replaced, without the complicated removal of the mechanism or cutting apart glued joints. This catalog describes the construction and function in detail in English and presents their use in the Solo, Duo and Tri-Phonola. We offer a PDF copy of this rare catalog (17 pages) for research purposes on request.