Anyway, this question caught my attention, so I did some more research and discovered that the first appearance of a computer in a cinema movie was probably a mockup named "EMERAC" (reminiscenting about ENIAC or UNIVAC?) in "Desk Set" with Katherine Hepburn from 1957. It even consisted of real computer parts, namely IBM 704 magnetic tapes and some panels from the famous SAGE military computer (based on vacuum tubes and ferrite core memory).

No list of computers in old movies would not be complete without HAL9000 from "A Space Odyssey". Timeless.
