Stamps of United States, 10 Jul 1973
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postage
1973, Jul 10 Progress in Electronics issue - electronics progress unwmk p. 11
| X | 6c | multi | Marconi spark coil & gap | |||
| X | 8c | multi | printed circuit board | |||
| X | 15c | multi | early microphone, radio tube, TV camera tube, radio speaker |
airmail
1973, Jul 10 Progress in Electronics issue
| X | 11c | multi | two De Forest audions |
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