Comment by CamperBob2

Disney was Hewlett and Packard's first customer in 1939, or was at least among the very first. The 200A oscillators were used to calibrate theater sound hardware for presentations of 'Fantasia' (although https://www.hewlettpackardhistory.com/item/a-deal-with-disne... says the Disney versions were designated 200B.)

Does it look like the one in the photo on that site? Sounds like a very cool artifact. You can't always tell much from the serial number -- for example, notice that they started their model numbers with '200,' just to make the company look bigger and more reputable.