400 / | HD | TYPEWRITING MACHINES |
679 | DF | MEANS AUXILIARY TO TYPEWRITING FUNCTION {12} |
689 | DF | .~ Means for muffling typewriter noise {1} |
690 | DF | .~.~ By sound-barrier enclosure for typewriter (e.g., by sound-absorbing material) {3} |
690.3 | .~.~.~ With externally protruding operating means |