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3855422
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Patent number 3,855,422
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Issue date Dec 17, 1974
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H04 - ELECTRIC COMMUNICATION TECHNIQUE
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3641273
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Patent number 3,641,273
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Issue date Feb 8, 1972
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H04 - ELECTRIC COMMUNICATION TECHNIQUE
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3629839
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Patent number 3,629,839
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Issue date Dec 21, 1971
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H04 - ELECTRIC COMMUNICATION TECHNIQUE
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3562431
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Patent number 3,562,431
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Issue date Feb 9, 1971
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H04 - ELECTRIC COMMUNICATION TECHNIQUE