I have the following message, received in response to an inquiry. You might want to contact him for some other sort of help, possibly in locating a wartime training film.

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I regret that the Naval History & Heritage Command does not have any 20mm guns available at the moment. Those in our collection are all currently on loan with other organizations.

Frank V. Thompson
Head, Curator Branch
Naval History & Heritage Command
Frank.Thompson@navy.mil.