Union Cipher Disk exhibit at the w:National Cryptologic Museum in 2005. Caption reads:

"Made of light yellow heavy card stock, the Union Cipher Disk was three and three-quarters inches (95 mm) in diameter. It consisted of two concentric disks of unequal size revolving on a central pivot. The disks were divided along their outer edges into thirty equal compartments. The smaller inner disk contained letters, terminations and word pauses, while the outer disk contained groups of signal numbers. For easier recognition, the number eight represented two. The initials A.J.M. represent the Chief Signal Officer General Albert J. Myer. Each disk had a control number used for accountability."

• Photographer: Austin Mills
• Image cropped from: [1]
• Licensing information: Email exchange with User:Matt Crypto

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This image, originally posted to Flickr, was reviewed on 29 May 2007 by the administrator or reviewer MECU, who confirmed that it was available on Flickr under the stated license on that date.