Fourth version 1969-1979 Black lens with no "50" on barrel, no focusing lock or lever.
第四版1969 - 1979，黑色镜筒上没有“50”字样，没有对焦锁和把手。
Vertical adjustment is accomplished using a lever with indications for matching the ceiling height, so there is no visual alignment required.
垂直调整通过指示杠杆适应上升限度，因而不需要任何目视调整。
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