English to Swedish Tech/Engineering Mechanics / Mech Engineering

plug är ett ord som förekommer i en bruksanvisning för en färgmärkningsmaskin i kapitlet Technical description i avsnittet Cover for Paint canister with agitator:

An air-actuated agitator (2) is located on the lid (1) of the paint canister. The lid also holds a suction pipe (5), which connects the canister with the paint pump via a hose as well as a connection, where the paint returns (6) from the machine. A plug (3) in the lid makes a visual inspection of the contents of the paint canister possible without having to remove the lid. The lid also contains a pipe (7) for the level sensor.
The agitator (2) consists of an air-actuated motor and on the output shaft a shaft with a propeller is mounted. The rotations of the motor are adjusted on the throttle valve (4).
A ball cock (8) is mounted on the suction pipe (5) for the shut-off of the suction pipe.

Men jag förstår inte hur en plugg eller propp skulle göra det lättare att visuellt inspektera innehållet i färgbehållaren.