This advertisement, reprinted from the American Chemical Society News Edition, may be the earliest advertisement for the DU model. The page is stamped Apr 25 1961 (though the date is crossed out) and the reprint note is circled.

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.