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| All Patents in class 181/138 | |||||
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| Picture | Number | Date | Patentee | Title | |
 | 218,707 | Aug. 19, 1879 | M.J. Maxwell, C.H. Billings | Improvement in mechanical telephones | |
| 243,214 | Jun. 21, 1881 | H.J. Colburn, I.W. Colburn | Acoustic telephone | ||