A tube is of slant to encourage the air entry. The level is adjustable by the nut that permits to adjust the height of the brass tank tube pushes.
The cork of the former models became rubber for the recent models (on the right)
Explanation on the constant level:
Oil in the glass reservoir is maintained by the air pressure. The out-flow is gotten only when oil is consumed and that the level lowers until the top of the tube (A point). Then a air bubble goes up in the vertical tube and the same quantity of oil is distributed.