Here is a Singer 15
And clone - Btw- that hole does not mean anything - just the way this one was made.
you can put the wrong bobbin case in, but it doesn't work.
Here is the clone with the singer case
Notice the notch the end of the "finger" hooks into, it's got one rather straight side and one slanted, that is the easiest way to tell which one you need, the slant side matches the angle of the finger.