This manual appears to have been a well-used desk copy. The stained cover provides evidence of secondary usage as a coaster.
The first Beckman Instruments DU Spectrophotometer was developed in 1940, when the company was still called National Technical Laboratories. Spurred by employee Howard Cary (who had previously been involved in pH meter development), NTL produced its first DU Spectrophotometer prototype in 1940. This first Model A contained a glass prism, while subsequent models used quartz crystal. The DU was initially developed under a U.S. government contract, but was ultimately used in many important scientific discoveries and developments of the 20th century, including penicillin and synthetic rubber research, petroleum refining, and the creation of explosives.
Beckman Instruments. “The Beckman Model DU Spectrophotometer and Accessories Instruction Manual,” 1954. Beckman Historical Collection, Box 104, Folder 197. Science History Institute. Philadelphia. https://digital.sciencehistory.org/works/r207tp471.