Whatever the person who put them there wants them to mean.
In westerns, the phrase is commonly used metaphorically with each notch on the gun representing one person the gunfighter owning it has killed.
They are for the choke wrench most likely. Choke markings are usually on the body of the tube.
The Life and Legend of Wyatt Earp - 1955 Nineteen Notches on His Gun 2-14 was released on: USA: 11 December 1956
To determine different shot patterns
Usually suppressors can be screwed on, so there should be notches to screw it on.
Goodnight Mrs. or Miss.
it means "goodnight, kisses"
Buenos noches means 'good night'
like puzzle pieces. They have notches that fit into gaps