The early system were known as open systems, because the oil-filler cap was open, or vented to the atmosphere.
早期的控制系统，就是人们所知道的“开式”系统，因为它的加机油管盖是开式的，或者说因其与大气连通。
Includes Low Head, Button Head, and Flat Head Screws, as well as special Socket Cap Screws such as Vented, Drilled Head, Self-Locking, and more.
包括低压头，半圆头及平头螺丝以及特殊内六角圆柱头螺钉如排空，钻孔头，自锁，等等。
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