Rock Island SW900m # 902 switching at Estherville, Iowa; September, 1974: Rock Island SW900m's originally were Winton-engined* EMC model SW's. The 902 was rebuilt in May, 1959 from SW # 504 which was built exactly 22 years earlier. RI had a fleet of 29 SW's built in 1937-38, and Wx4 staff cannot think of any other railroad rostering that many examples of one model at that early date in the diesel era. SP didn't even buy its first diesel until 1939, for example. The rebuild included a new body and frame, so there's nothing external to identify their provenance. This info comes from Louis A. Marre's Rock Island Diesel Locomotives, 1930-1980; Railfax Inc., 1982.
*The Smithsonian claims that their Winton engine, on dispaly at their Museum of History, is the only one known to still exist. Pretty amazing, eh? Of course there's also only one E-7 left, so there you go.