Doing It The Hard Way

I am willing to bet that the accepted gunsmith way of cutting the notch into the ejector isn't the way it was done when making military guns.

The gunsmith way is a bunch of careful measuring and duplicating the old notch in the new part and lots of installing the ejector and making sure the notch lines up with the hole.

Wanna bet that the original way of making that notch was to fixture the frame so that the hole was lined up with a drill bit or end mill, the ejector installed and then the drill was passed through the hole in the frame, cutting the notch for the pin?

Or that the hole in the frame for the pin was drilled with the ejector installed?

Why do I bet this?

Because it's a process that makes sense on a production line where every part is made to be identical and interchangeable.