Original old building vintage catalog ROBERTS AND SCHAEFER COMPANY of Chicago, IL.  Barrel Shell Roofs, Shell Domes.  Z-D System of Permanent Monolithic Reinforced Concrete Construction.  Disbound from a large 1938 construction book, this unique one page roofing technical data specification sheet would display wonderfully matted and framed. It would be a marvelous addition to your architecture and engineering library.